stagefright: add indexed buffer and format getters to MediaCodec

These are designed to be called from the same thread as the one
calling dequeue?Buffer, and use a mutex to avoid switching
context.  All other calls of MediaCodec are designed to be blocking
and synchronous.

Bug: 14297827
Change-Id: If341c6e4407ca6f10f5e0d47008dddc0e20b0a50