bluesleep: Add support for Bluedroid

 * Bluedroid doesn't use HCI for signalling, so we'll add a procfs
   interface that configures low-power modes.
 * Set BT_WAKE_VIA_PROC to use this with Bluedroid.
 * Based on work from NVidia.

Change-Id: I705145b36ad1a48ba128fa41eb73967fd1d0d027