MergingMediaPeriod + subtitle fixes

- Correctly null out streams[j] in the case that a renderer
  is being disabled.
- Read discontinuities from all children, not just enabled
  ones. This fixes a failure when reading a discontinuity
  with all renderers disabled.
- Add in some assertions to make incorrect stream selection
  failures obvious and immediate.
- Relocate subtitles so they're above the shutter (needed so
  they continue to be visible when video is disabled but
  text is still enabled).

Issue: #1854

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135089944

library/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java

@@ -1264,11 +1264,14 @@ import java.io.IOException;
           sampleStreams, streamResetFlags, positionUs);
       periodTrackSelections = trackSelections;
 
+      // Update whether we have enabled tracks and sanity check the expected streams are non-null.
       hasEnabledTracks = false;
       for (int i = 0; i < sampleStreams.length; i++) {
         if (sampleStreams[i] != null) {
+          Assertions.checkState(trackSelections.get(i) != null);
           hasEnabledTracks = true;
-          break;
+        } else {
+          Assertions.checkState(trackSelections.get(i) == null);
         }
       }
 

library/src/main/java/com/google/android/exoplayer2/source/MergingMediaPeriod.java

@@ -17,6 +17,7 @@ package com.google.android.exoplayer2.source;
 
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.trackselection.TrackSelection;
+import com.google.android.exoplayer2.util.Assertions;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.IdentityHashMap;
@@ -103,10 +104,17 @@ import java.util.IdentityHashMap;
       boolean periodEnabled = false;
       for (int j = 0; j < selections.length; j++) {
         if (selectionChildIndices[j] == i) {
+          // Assert that the child provided a stream for the selection.
+          Assertions.checkState(childStreams[j] != null);
           streams[j] = childStreams[j];
-          if (childStreams[j] != null) {
-            periodEnabled = true;
-            streamPeriodIndices.put(childStreams[j], i);
+          periodEnabled = true;
+          streamPeriodIndices.put(childStreams[j], i);
+        } else if (streamChildIndices[j] == i) {
+          // Assert that the child cleared any previous stream.
+          Assertions.checkState(childStreams[j] == null);
+          if (selectionChildIndices[j] == C.INDEX_UNSET) {
+            // No other child will be setting the stream at index j, so clear it.
+            streams[j] = null;
           }
         }
       }
@@ -133,21 +141,22 @@ import java.util.IdentityHashMap;
 
   @Override
   public long readDiscontinuity() {
-    long positionUs = enabledPeriods[0].readDiscontinuity();
+    long positionUs = periods[0].readDiscontinuity();
+    // Periods other than the first one are not allowed to report discontinuities.
+    for (int i = 1; i < periods.length; i++) {
+      if (periods[i].readDiscontinuity() != C.TIME_UNSET) {
+        throw new IllegalStateException("Child reported discontinuity");
+      }
+    }
+    // It must be possible to seek enabled periods to the new position, if there is one.
     if (positionUs != C.TIME_UNSET) {
-      // It must be possible to seek additional periods to the new position.
-      for (int i = 1; i < enabledPeriods.length; i++) {
-        if (enabledPeriods[i].seekToUs(positionUs) != positionUs) {
+      for (int i = 0; i < enabledPeriods.length; i++) {
+        if (enabledPeriods[i] != periods[0]
+            && enabledPeriods[i].seekToUs(positionUs) != positionUs) {
           throw new IllegalStateException("Children seeked to different positions");
         }
       }
     }
-    // Additional periods are not allowed to report discontinuities.
-    for (int i = 1; i < enabledPeriods.length; i++) {
-      if (enabledPeriods[i].readDiscontinuity() != C.TIME_UNSET) {
-        throw new IllegalStateException("Child reported discontinuity");
-      }
-    }
     return positionUs;
   }
 

library/src/main/res/layout/exo_simple_player_view.xml

@@ -22,17 +22,17 @@
       android:layout_height="match_parent"
       android:layout_gravity="center">
 
+    <View android:id="@+id/shutter"
+      android:layout_width="match_parent"
+      android:layout_height="match_parent"
+      android:background="@android:color/black"/>
+
     <com.google.android.exoplayer2.ui.SubtitleView android:id="@+id/subtitles"
         android:layout_width="match_parent"
         android:layout_height="match_parent"/>
 
   </com.google.android.exoplayer2.ui.AspectRatioFrameLayout>
 
-  <View android:id="@+id/shutter"
-      android:layout_width="match_parent"
-      android:layout_height="match_parent"
-      android:background="@android:color/black"/>
-
   <com.google.android.exoplayer2.ui.PlaybackControlView android:id="@+id/control"
       android:layout_width="match_parent"
       android:layout_height="match_parent"/>