AlarmManagerService: remove legacy /dev/alarm support

We've removed the Android alarm driver from our supported kernels.  It's
time to remove the userspace callers too: they make the native side of
AlarmManagerService more complex, and send the wrong signal about
/dev/alarm still being supported.

Bug: 28357356

Change-Id: I225d76c3f4e695a831d4419e050e8e178e1224b5
Signed-off-by: Greg Hackmann <ghackmann@google.com>