uptime: Use clock_gettime to get monotonic time

The first field in /proc/uptime is bootbased time, not monotonic
time.  If the kernel tracks bootbased time correctly, it counts
elapsed run time as well as sleep time, which is the same as the
elapsed time in the android alarm driver, and sleep time is
always returned as 0.

Use clock_gettime(CLOCK_MONOTONIC) instead, which will return
elapsed run time not counting sleep time on all platforms.

Change-Id: I28a22e8c93d78f62666ee8c877c7c6718a2b640a