Delay handleEnable() until Bluetooth state is OFF

During shutdown of the Bluetooth stack, if a request to
enable Bluetooth is received, the request is processed immediately.
As a result, we don't let the Bluetooth service to shutdown cleanly,
and the Bluetooth process with the native backend doesn't go away.
This creates various issues in the native backend stack.

Now, after an enable request is received, if necessary we delay
acting on it, until the Bluetooth state is OFF.

Bug: 17723234
Change-Id: I55db4bbd4f45227aa49eae996dcc328f76557a9b