Remove one redundant frame copy in MetadataRetriever

When extracting bitmaps, MediaMetadataRetriever does a copy from
StagefrightMetadataRetriever's VideoFrame to IMemory. We could
just allocate IMemory directly and return that to the client.

bug: 78475896
Change-Id: Ibe07e7d2c68f031261470308476089c2fa9298ea