Transcoder: Fix video track transcoding crash.

The VideoTrackTranscoder had a bug where the encoder could
outlive the transcoder object, because the encoder owns the
output buffers. This caused a crash when the encoder sent async
callbacks to the transcoder after it had been released. This fix
gives the encoder a weak reference to the transcoder and gives
outstanding buffers a strong reference to the encoder.

Fixes: 160711746
Test: Transcoder unit tests (build_and_run_all_unit_tests.sh).
Test: New unit test to trigger the crash before the fix.
Change-Id: I8141591399b6cff642d2d322809d3254adbefaaf
diff --git a/media/libmediatranscoding/transcoder/MediaTrackTranscoder.cpp b/media/libmediatranscoding/transcoder/MediaTrackTranscoder.cpp
index b485826..e3996b0 100644
--- a/media/libmediatranscoding/transcoder/MediaTrackTranscoder.cpp
+++ b/media/libmediatranscoding/transcoder/MediaTrackTranscoder.cpp
@@ -94,6 +94,7 @@
     if (mState == STARTED) {
         abortTranscodeLoop();
         mTranscodingThread.join();
+        mOutputQueue->abort();  // Wake up any threads waiting for samples.
         mState = STOPPED;
         return true;
     }