Protect against potential endless loop in BTU task timer handling

Fix potential bug where stale timer list entry causes endless loops in
BTU task. Also added return parameter to GKI_remove_from_timer_list() to
allow breaking out of the BTU timer task look in case the timer list
becomes corrupted.

Bug: 16897789
Change-Id: Ic70cf4346efbb063bbb952ebe7c2f7d0bf395493