Parse uid_cputime proc file

Instead of using the ProcessCpuTracker as the source of truth, we
periodically poll the new uid_cputime kernel module for stats
on all UID cpu time.

TODO: Need to tell the kernel when to stop tracking UIDs (aka on package
uninstall).

Change-Id: Id1d251aae23ab53f7acc0aba3bca5118bc2c194a