[Android][Extension] Implement W3C Audio/Video codecs API [PART 2]

The current implementation relies on the MediaCodec class introduced
since Android 4.1 (API Level 16), as a result, an abstract class XWalkMediaCodec
is added to allow this API run on the older Android but will empty list.

The support for 4.0 (API level == 15) will be added in another PR.

SPEC=http://www.w3.org/2012/sysapps/device-capabilities/
BUG=https://crosswalk-project.org/jira/browse/XWALK-48