Device cannot go to sleep while connected to HBM-570 BT headset

The headset never disconnect the Obex connection for PBAP,
so in this case, we never release the wakelock.
Our PBAP implementation won't release wakelock
until the Obex is disconnected by the remote device
To better control the wake lock, we should
only acquire the wakelock when we receive the reqeust from the remote device
will release the wakelock after we send the response packets.

bug:10957699
Change-Id: I85e0609e36788d9c36334dd804ec52c90b90615b