Bug 976172 - Part 2 Enable audio offloading in gecko r=padenot

Implements a new class AudioOffloadPlayer to make use of
Android::AudioTrack class's offloading capability
Bug 976172 - Part 1 Changes in MediaOmxDecoder to enable audio offloading r=roc

This adds support in MediaOmxDecoder to dynamically switch between
MediaDecoderStateMachine and AudioOffloadPlayer