SystemClock: use clock_gettime() on devices without /dev/alarm

On devices with an up-to-date kernel, the back-in-time bug affecting
clock_gettime() has been fixed and it can safely be used as an
alternative to the ANDROID_ALARM_GET_TIME ioctl.  To ensure consistent
behavior on existing devices, make clock_gettime() a fallback for when
the alarm driver isn't available.

Change-Id: I384af5e7ec9e73e0bad4b6b0f987a8ea4583cba6
Signed-off-by: Greg Hackmann <ghackmann@google.com>