Slightly Performance Tuning

1. Enlarge memory heap to decrease the frequency of AudioTrack
releaseBuffer() called, which would lead to a sudden pause during the
play.
2. In Android manifest file, enable hardware acceleration. Note that
this simply means to render bitmap by hardware.
3. Enlarge socket receiving buffer size to decrease the frequency of
broken image due to lost of packet.
4. Raise the priority of FFmpeg thread.