Change prepare() for maybePrepare() in HlsSampleStreamWrapper

This will allow asynchronous preparation.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=136176854

library/src/main/java/com/google/android/exoplayer2/source/hls/HlsMediaPeriod.java

@@ -328,7 +328,7 @@ import java.util.List;
       sampleStreamWrappers = new HlsSampleStreamWrapper[] {
           buildSampleStreamWrapper(C.TRACK_TYPE_DEFAULT, baseUri, variants, null, null)};
       pendingPrepareCount = 1;
-      sampleStreamWrappers[0].prepare();
+      sampleStreamWrappers[0].continuePreparing();
       return;
     }
 
@@ -369,16 +369,16 @@ import java.util.List;
       selectedVariants.toArray(variants);
       HlsSampleStreamWrapper sampleStreamWrapper = buildSampleStreamWrapper(C.TRACK_TYPE_DEFAULT,
           baseUri, variants, masterPlaylist.muxedAudioFormat, masterPlaylist.muxedCaptionFormat);
-      sampleStreamWrapper.prepare();
       sampleStreamWrappers[currentWrapperIndex++] = sampleStreamWrapper;
+      sampleStreamWrapper.continuePreparing();
     }
 
     // Build audio stream wrappers.
     for (int i = 0; i < audioVariants.size(); i++) {
       HlsSampleStreamWrapper sampleStreamWrapper = buildSampleStreamWrapper(C.TRACK_TYPE_AUDIO,
           baseUri, new HlsMasterPlaylist.HlsUrl[] {audioVariants.get(i)}, null, null);
-      sampleStreamWrapper.prepare();
       sampleStreamWrappers[currentWrapperIndex++] = sampleStreamWrapper;
+      sampleStreamWrapper.continuePreparing();
     }
 
     // Build subtitle stream wrappers.

library/src/main/java/com/google/android/exoplayer2/source/hls/HlsSampleStreamWrapper.java

@@ -144,8 +144,10 @@ import java.util.LinkedList;
     pendingResetPositionUs = positionUs;
   }
 
-  public void prepare() {
-    continueLoading(lastSeekPositionUs);
+  public void continuePreparing() {
+    if (!prepared) {
+      continueLoading(lastSeekPositionUs);
+    }
   }
 
   /**
@@ -154,7 +156,8 @@ import java.util.LinkedList;
    */
   public void prepareSingleTrack(Format format) {
     track(0).format(format);
-    endTracks();
+    sampleQueuesBuilt = true;
+    maybeFinishPrepare();
   }
 
   public void maybeThrowPrepareError() throws IOException {