Fix single sample handling ProgressiveMediaPeriod when disabling tracks

When deselecting the single sample track and later re-selecting this
track, the current shortcuts in ProgressiveMediaPeriod don't handle
this case correctly and cause assertion failures.

In particular, this change fixes 3 issues:
 1. When re-selecting the single sample track, we have cleared the
    SampleQueue and need to reload the sample. The existing shortcut
    should only be applied to avoid the reload when starting from a
    non-zero position.
 2. When de-selecting the track, ProgressiveMediaPeriod is left in
    an inconsistent state where the sample queues are empty but
    loadingFinished is still true. Fix this by resetting
    loadingFinished to false.
 3. When seeking, we avoid reloading the stream if we can keep
    inside the existing samples. This logic assumes that all
    remaining samples will continue to be loaded in the queue.
    This condition isn't true though for single sample tracks
    that have been de-selected. They appear to support the seek
    inside the queue (=no seek necessary, always supported), but
    still require a new load if there is no ongoing one to load
    the sample. Fix this by checking this implicit assumption
    (still loading, or loading finished).

PiperOrigin-RevId: 642650248

Author: tonihei

Diff for: libraries/exoplayer/src/main/java/androidx/media3/exoplayer/source/ProgressiveMediaPeriod.java

@@ -276,11 +276,11 @@ public long selectTracks(
         streams[i] = null;
       }
     }
-    // We'll always need to seek if this is a first selection to a non-zero position, or if we're
-    // making a selection having previously disabled all tracks, except for when we have a single
-    // sample.
+    // We'll always need to seek if this is a first selection to a non-zero position (except for
+    // when we have a single sample only), or if we're making a selection having previously
+    // disabled all tracks.
     boolean seekRequired =
-        !isSingleSample && (seenFirstTrackSelection ? oldEnabledTrackCount == 0 : positionUs != 0);
+        seenFirstTrackSelection ? oldEnabledTrackCount == 0 : positionUs != 0 && !isSingleSample;
     // Select new tracks.
     for (int i = 0; i < selections.length; i++) {
       if (streams[i] == null && selections[i] != null) {
@@ -315,6 +315,7 @@ public long selectTracks(
         }
         loader.cancelLoading();
       } else {
+        loadingFinished = false;
         for (SampleQueue sampleQueue : sampleQueues) {
           sampleQueue.reset();
         }
@@ -434,8 +435,10 @@ public long seekToUs(long positionUs) {
       return positionUs;
     }
 
-    // If we're not playing a live stream, try and seek within the buffer.
+    // If we're not playing a live stream, and when loading will continue (or has finished), try
+    // and seek within the existing buffer instead of restarting the load.
     if (dataType != C.DATA_TYPE_MEDIA_PROGRESSIVE_LIVE
+        && (loadingFinished || loader.isLoading())
         && seekInsideBufferUs(trackIsAudioVideoFlags, positionUs)) {
       return positionUs;
     }