Revert "Prefer ANDROID_ALARM_ELAPSED_REALTIME over CLOCK_MONOTONIC"

This reverts commit 668a31676fc65bd8686f8b4a11e25753cf8bc55d.
[timed] Prefer CLOCK_BOOTTIME over CLOCK_MONOTONIC

The CLOCK_MONOTONIC might get frozen during suspend.
CLOCK_BOOTTIME does not have this problem and should be used
when available.
Merge pull request #40 from pgerdt/use-clock-boottime

Prefer CLOCK_BOOTTIME over CLOCK_MONOTONIC, do not use ANDROID_ALARM_ELAPSED_REALTIME