BUG=webrtc:6649
- Supports Bluetooth Headset profile.
- Detects new BT headset:
+ enabled at start, and
+ powered on during active call.
- Enables/disables BT SCO channel when BT device is selected.
- Removes proximity sensor usage to avoid conflicts (will be added again later).
- Adds new (unused) APIs to explicitly select audio device.
- Starts routing audio to BT headset when enabled, i.e, BT is default.
Review-Url: https://codereview.webrtc.org/2501983002
Cr-Commit-Position: refs/heads/master@{#15610}
c3c2f31852