Transcoder: Add support for pausing transcoding on a sync frame.

- Added support for stopping transcoders on a sync frame.
- Refactored MediaTrackTranscoders and MediaSampleWriter to stop()
asynchronously.
- Fixed callback and error handling logic in MediaTranscoder.
- Added tests for pause and stopping on sync frame.

Bug: 162886306
Test: Unit tests.

Change-Id: If689a10dfee198c674c4c13b865a7c56a901e075
diff --git a/media/libmediatranscoding/transcoder/MediaSampleReaderNDK.cpp b/media/libmediatranscoding/transcoder/MediaSampleReaderNDK.cpp
index 53d567e..d2f6c40 100644
--- a/media/libmediatranscoding/transcoder/MediaSampleReaderNDK.cpp
+++ b/media/libmediatranscoding/transcoder/MediaSampleReaderNDK.cpp
@@ -181,6 +181,11 @@
     if (mEosReached) {
         return AMEDIA_ERROR_END_OF_STREAM;
     }
+
+    if (!mEnforceSequentialAccess) {
+        return moveToTrack_l(trackIndex);
+    }
+
     return AMEDIA_OK;
 }