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/benchmark/MediaTrackTranscoderBenchmark.cpp b/media/libmediatranscoding/transcoder/benchmark/MediaTrackTranscoderBenchmark.cpp
index aee0ed6..351d80b 100644
--- a/media/libmediatranscoding/transcoder/benchmark/MediaTrackTranscoderBenchmark.cpp
+++ b/media/libmediatranscoding/transcoder/benchmark/MediaTrackTranscoderBenchmark.cpp
@@ -61,6 +61,12 @@
         mCondition.notify_all();
     }
 
+    virtual void onTrackStopped(const MediaTrackTranscoder* transcoder __unused) override {
+        std::unique_lock lock(mMutex);
+        mFinished = true;
+        mCondition.notify_all();
+    }
+
     virtual void onTrackError(const MediaTrackTranscoder* transcoder __unused,
                               media_status_t status) override {
         std::unique_lock lock(mMutex);