Transcoder: Added MediaTranscoder and unit test.

MediaTranscoder is the API for the native transcoding library.

Test: Unit tests.
Bug: 156003955, 152091443, 155918341
Change-Id: I24b52d174db0faecea8f331ef6d8a3dc4e473c4e
diff --git a/media/libmediatranscoding/transcoder/tests/TrackTranscoderTestUtils.h b/media/libmediatranscoding/transcoder/tests/TrackTranscoderTestUtils.h
index 6b9131c..79c227b 100644
--- a/media/libmediatranscoding/transcoder/tests/TrackTranscoderTestUtils.h
+++ b/media/libmediatranscoding/transcoder/tests/TrackTranscoderTestUtils.h
@@ -56,13 +56,13 @@
     ~TestCallback() = default;
 
     // MediaTrackTranscoderCallback
-    void onTrackFinished(MediaTrackTranscoder* transcoder __unused) {
+    void onTrackFinished(const MediaTrackTranscoder* transcoder __unused) {
         std::unique_lock<std::mutex> lock(mMutex);
         mTranscodingFinished = true;
         mCv.notify_all();
     }
 
-    void onTrackError(MediaTrackTranscoder* transcoder __unused, media_status_t status) {
+    void onTrackError(const MediaTrackTranscoder* transcoder __unused, media_status_t status) {
         std::unique_lock<std::mutex> lock(mMutex);
         mTranscodingFinished = true;
         mStatus = status;