Fall back to CLOCK_BOOTTIME if CLOCK_BOOTTIME_ALARM fails

If the cuttlefish device does not have an rtc device (such as the crosvm
VMM) the bt osi layer can promote crashes due to it not being able to
create a CLOCK_BOOTTIME_ALARM timer. Bring back a fallback but enable it
at runtime instead of compile time.

Bug: 126955943
Test: run with cuttlefish
Change-Id: I3ab0282b3e8fde776aa7b37d5772c8f62cf957bf