A2DP-SINK: Use MessageLoopThread for decoding

* Instead of OSI thread, use MessageLoopThread for decoding
* Similar to A2DP source
* Change tBTIF_A2DP_SINK_CB into a proper class BtifA2dpSinkControlBlock
* Initialize variables in BtifA2dpSinkControlBlock's constructor
* Free un-freed variables in BtifA2dpSinkControlBlock::Reset() method

Bug: 110303473
Test: mm -j40, unit test, stream music
Change-Id: I21f5b0c9f88d2cadada6fc854070d31ac378244f