Purge the android_alarm headers.

These aren't part of the current UAPI headers. They're already gone
from the unified headers, but it would be good to remove them from
the legacy headers as well since it will help find the people that
are still using them (if any).

Test: ndk/checkbuild.py
Bug: None
Change-Id: I04f78e06116a7fba3c894c710dee014cda4030f4