Check adaptive playback support via CodecCapabilities

Added flags field to CodecCapabilities, so that applications
can query whether codecs support various features.  For now
added one video-decoder feature:

kFlagSupportsAdaptivePlayback

Media playback applications can query it to see if a codec
supports seamless resolution changes, such as by supporting
dynamic output buffers.

Signed-off-by: Lajos Molnar <lajos@google.com>
Change-Id: I09da46e8ab6b8645fa2749e33128e49eda2e865b
Related-to-bug: 7093648