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

Test: cts-tradefed run singleCommand cts -d -c android.app.cts.AlarmManagerTest
Change-Id: I619e2d8a98e4c4728131a2d826b1295f858abedd
Signed-off-by: Greg Hackmann <ghackmann@google.com>