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/MediaTrackTranscoder.cpp b/media/libmediatranscoding/transcoder/MediaTrackTranscoder.cpp
index 3a281ba..b485826 100644
--- a/media/libmediatranscoding/transcoder/MediaTrackTranscoder.cpp
+++ b/media/libmediatranscoding/transcoder/MediaTrackTranscoder.cpp
@@ -102,6 +102,12 @@
     return false;
 }
 
+void MediaTrackTranscoder::notifyTrackFormatAvailable() {
+    if (auto callbacks = mTranscoderCallback.lock()) {
+        callbacks->onTrackFormatAvailable(this);
+    }
+}
+
 std::shared_ptr<MediaSampleQueue> MediaTrackTranscoder::getOutputQueue() const {
     return mOutputQueue;
 }