Adds basic Bluetooth support to AppRTCMobile

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-Original-Commit-Position: refs/heads/master@{#15610}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: c3c2f318521270ac42e3721d707259a29288e98c
7 files changed