Fix GKI_add_to_timer_list to handle middle-of-list insertions correctly

The newly added timer_list code didn't handle middle-of-list insertions
correctly, leading to problems pairing certain Bluetooth devices as well
as causing 100% CPU usage for the Bluetooth task.

Change-Id: I3d4c22b775a91da2a3f983e8c78facbd91d4cb7d