BT Interface: fix hci cmd timeout

Add mutex in function Send() and OnTimeout() to do mutual
exclusive access on lpm_wake_deasserted.

If lpm_wake_deasserted is out of sync with the actual
wake state, then platforms which use the lpm hint
will break.

Bug: 64299848
Test: run affected platform for a long period of time
and find that hci commands do not time out
Change-Id: I9f630d6f40f84b9c047c6488632cd44d6602bb14