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-Commit-Position: refs/heads/master@{#15610}

webrtc/examples/BUILD.gn

@@ -65,6 +65,7 @@ if (is_android) {
   android_library("AppRTCMobile_javalib") {
     java_files = [
       "androidapp/src/org/appspot/apprtc/AppRTCAudioManager.java",
+      "androidapp/src/org/appspot/apprtc/AppRTCBluetoothManager.java",
       "androidapp/src/org/appspot/apprtc/AppRTCClient.java",
       "androidapp/src/org/appspot/apprtc/AppRTCProximitySensor.java",
       "androidapp/src/org/appspot/apprtc/CallActivity.java",