Fixing A2dpSinkStreamingStateMachine thread leak.

A2dpSinkStreamingStateMachine was being created on every Connection
leading to a leak. Fix is to Quit the A2dpSinkStreamingStateMachine
on A2dpSinkStateMachine's Quit() and create a new Streaming
StateMachine only if it is null.
Note that both the Sink and SinkStreaming State machines quit only
on Adapter off, not on BT disconnect.

Bug: b/32097580
Tested: On Mojave and Angler with bat_mas-userdebug

Change-Id: I7f2e3ac01ce5ff965afe407e3605cac00497af36
(cherry picked from commit af41a720081dd4e9d12e1b3f082e9202ae84252d)