Remove a suppression introduced by the checker-framework 3.3.0 upgrade

Update the comment on a suppression of Field.get(null) which is safe
but blocked by the checker.

These suppressions were introduced in https://github.com/google/ExoPlayer/commit/d1e057244812a566a5fc8b18a2caec1908db1a43

PiperOrigin-RevId: 307036441

extensions/cronet/src/main/java/com/google/android/exoplayer2/ext/cronet/CronetEngineWrapper.java

@@ -166,7 +166,7 @@ public final class CronetEngineWrapper {
     private final boolean preferGMSCoreCronet;
 
     // Multi-catch can only be used for API 19+ in this case.
-    // incompatible types in argument.
+    // Field#get(null) is blocked by the null-checker, but is safe because the field is static.
     @SuppressWarnings({"UseMultiCatch", "nullness:argument.type.incompatible"})
     public CronetProviderComparator(boolean preferGMSCoreCronet) {
       // GMSCore CronetProvider classes are only available in some configurations.

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

@@ -96,8 +96,6 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
     return Assertions.checkNotNull(trackGroups);
   }
 
-  // unboxing a possibly-null reference streamPeriodIndices.get(streams[i])
-  @SuppressWarnings("nullness:unboxing.of.nullable")
   @Override
   public long selectTracks(
       @NullableType TrackSelection[] selections,
@@ -109,8 +107,8 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
     int[] streamChildIndices = new int[selections.length];
     int[] selectionChildIndices = new int[selections.length];
     for (int i = 0; i < selections.length; i++) {
-      streamChildIndices[i] = streams[i] == null ? C.INDEX_UNSET
-          : streamPeriodIndices.get(streams[i]);
+      Integer streamChildIndex = streams[i] == null ? null : streamPeriodIndices.get(streams[i]);
+      streamChildIndices[i] = streamChildIndex == null ? C.INDEX_UNSET : streamChildIndex;
       selectionChildIndices[i] = C.INDEX_UNSET;
       if (selections[i] != null) {
         TrackGroup trackGroup = selections[i].getTrackGroup();