transcoder: wait for actual encoder output format to start writer

bug: 154734285
bug: 158240484

test: unit tests. In particular:

run MediaTranscoderTests and MediaTranscodingServiceRealTests, and
check the output files. Should play with proper aspect ratio on
host and device.

Change-Id: I8254fd8bd42678a414cab5fb0f61c4ba56d6c114
diff --git a/media/libmediatranscoding/transcoder/PassthroughTrackTranscoder.cpp b/media/libmediatranscoding/transcoder/PassthroughTrackTranscoder.cpp
index 7806208..a4cbf33 100644
--- a/media/libmediatranscoding/transcoder/PassthroughTrackTranscoder.cpp
+++ b/media/libmediatranscoding/transcoder/PassthroughTrackTranscoder.cpp
@@ -96,6 +96,9 @@
     MediaSampleInfo info;
     std::shared_ptr<MediaSample> sample;
 
+    // Notify the track format as soon as we start. It's same as the source format.
+    notifyTrackFormatAvailable();
+
     MediaSample::OnSampleReleasedCallback bufferReleaseCallback =
             [bufferPool = mBufferPool](MediaSample* sample) {
                 bufferPool->returnBuffer(const_cast<uint8_t*>(sample->buffer));