Transcoder: Added MediaSampleWriter and unit tests.

MediaSampleWriter pulls samples from its input sample queues,
in time interleaved order, and adds them to its muxer.

Test: Unit test (build_and_run_all_unit_tests.sh).
Bug: 156004594
Change-Id: I7f0085e9ef6ec50dca7d30c6a86709b961056d1b
diff --git a/media/libmediatranscoding/transcoder/Android.bp b/media/libmediatranscoding/transcoder/Android.bp
index 7f6630f..0b7ddbb 100644
--- a/media/libmediatranscoding/transcoder/Android.bp
+++ b/media/libmediatranscoding/transcoder/Android.bp
@@ -20,6 +20,7 @@
     srcs: [
         "MediaSampleQueue.cpp",
         "MediaSampleReaderNDK.cpp",
+        "MediaSampleWriter.cpp",
         "MediaTrackTranscoder.cpp",
         "PassthroughTrackTranscoder.cpp",
         "VideoTrackTranscoder.cpp",