Bluetooth: Pbap: Acquire wakelocks only when required

This patch updates the wakelock acquiring logic in PBAP Server only when
OBEX transaction is being done by PBAP Client. Without this patch it was
observed that wakelock was acquired during obex server connection
establishement and was only released when connection was disconnected
which was resulting in extra power consumption when PBAP connection was
idle.

Change-Id: I3189036649f09c6df944d575eabb5b03863ee7d7
CRs-Fixed: 565528
(cherry picked from commit c586750631298660bcbcb4587ddc1233648527da)
(cherry picked from commit 366ab6df5821bf4a9593ff1cdbe353d487a8609f)