Bug 1531833 - Add bluetooth permission to GeckoView. r=snorp

This is needed to open an SCO channel and do proper (low-latency) bluetooth
communication when doing a call using WebRTC, or simply recording local audio in
a web application.

I think this is more of a GeckoView thing, but I'm a bit fuzzy on the
distinction, maybe it's the wrong manifest. I tested using Fennec.

Differential Revision: https://phabricator.services.mozilla.com/D21734

--HG--
extra : moz-landing-system : lando

mobile/android/geckoview/src/main/AndroidManifest.xml

@@ -5,6 +5,7 @@
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
     <uses-permission android:name="android.permission.INTERNET"/>
     <uses-permission android:name="android.permission.WAKE_LOCK"/>
+    <uses-permission android:name="android.permission.MODIFY_AUDIO_SETTINGS" />
 
     <uses-feature
             android:name="android.hardware.location"