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/tests/Android.bp b/media/libmediatranscoding/transcoder/tests/Android.bp
index 52a7a71..926110885 100644
--- a/media/libmediatranscoding/transcoder/tests/Android.bp
+++ b/media/libmediatranscoding/transcoder/tests/Android.bp
@@ -67,3 +67,10 @@
     srcs: ["PassthroughTrackTranscoderTests.cpp"],
     shared_libs: ["libcrypto"],
 }
+
+// MediaSampleWriter unit test
+cc_test {
+    name: "MediaSampleWriterTests",
+    defaults: ["testdefaults"],
+    srcs: ["MediaSampleWriterTests.cpp"],
+}