[timed] Prefer ANDROID_ALARM_ELAPSED_REALTIME over CLOCK_MONOTONIC

The CLOCK_MONOTONIC might get frozen during suspend. The android
specific ANDROID_ALARM_ELAPSED_REALTIME does not have this problem
and should be used when available.
Merge pull request #32 from pgerdt/use-android-monotonic-time-source

[timed] Prefer ANDROID_ALARM_ELAPSED_REALTIME over CLOCK_MONOTONIC