Track the global setting of BT A2DP on/off

The default phone speaker, bluetooth audio devices, and wired audio
devices are handled as system audio routes. When an app starts to
make sound to a system audio route, the system audio route will be
selected with the following order.

1. The slected route by the app after the latest connection of
   an audio device.
2. The lastly connected audio device.
3. The default phone speaker if not connected.

Bug: 64811580
Test: Passed MediaRouterTest, manual
Change-Id: I77badb14596bc661c44237957504164f5476e595