libbt: Add btlock support

 * Concurrent WiFi/BT firmware loading can cause a chip crash.
 * Kernel supports a btlock device which is a simple semaphore that can
   protect from this situation.
 * Add support in the Bluedroid vendor lib for this on the BT side, as
   WiFi is already done directly in the kernel.

Change-Id: I8b9d3b1ccd06cc5d5528c7d6d685048c258d9887