MediaMetadataRetriever: add static lock for image extraction

MediaMetadataRetriever binder service has a static lock in
MetadataRetrieverClient. The static lock serializes getFrameAtTime(),
getImageAtIndex(), getImageRectAtIndex() and getFrameAtIndex()
regardless of where the transaction being originated.

The static lock in the service side throttles image extraction not to
consume too much memory. But the static lock can cause to consume all
the binder thread pools and lead to starvation.

By adding a static lock to the client side(MediaMetadataRetriever),
image extraction from the same process will consume only one thread in
the thread pool.

Flag: EXEMPT bugfix
Bug: 359802236
Test: presubmit
Change-Id: I8476df2aac34506d7571b98e1c24aee8d74a7900