Use local lock for most MediaMetadataRetriever methods. Bug 2560834.

MediaMetadataRetriever uses a single static lock for all operations.
This effectively serializes all metadata retrieval operations in a
single process. This patch uses the object level lock for all normal
operations and only uses the static lock to serialize calls to
release.

Change-Id: I81c9f234c2f0007a26d18e1398c709b41a4dbbd7