ACodec: Add custom MediaFormat keys.

that can be set from java/native while configuring
the MediaCodec:
 [1] "prefer-adaptive-playback" to prefer
     adaptive-playback over dynamic buffering
 [2] "prevent-screen-capture" to prevent screen
     capture of (non-L1) protected content

Conflicts:
media/libstagefright/ACodec.cpp
Change-Id: Ic02fc9f1d68fd9803ec154498a86e2537efcce85