Retry connectAudio if it returns false.

Sometimes, when BluetoothManager#connectBluetoothAudio is called
immediately after a bluetooth device connects, the HeadsetStateMachine
in the bluetooth stack is not yet ready to accept commands, and
therefore BluetoothHeadset#connectAudio returns false. This change adds
a retry with 500ms backoff.

Part of the fix for b/22507913

Bug: 22507913
Change-Id: I3084496c3af74687ea66c8558e20f1260420381c