FP2-1887: bluetooth: Use /dev/alarm (if it exists) for monotonic us time

bluetooth: Use /dev/alarm (if it exists) for monotonic us time

This matches what is done in system/core/libutils to get the
current time in usecs.

On some platforms, I am seeing wildly different values returned by
clock_gettime(CLOCK_BOOTTIME) (presumably based on which CPU is
being queried).  These times vary by 5 seconds and therefore
make an unreliable clock to use for counting ticks.

Change-Id: Id970b708fcefc50e16f4efa0f843cb630db4757f
1 file changed