uid_cputime: fix cputime overflow

Converting cputime_t to usec caused overflow when the value is greater
than 1 hour. Use msec and convert to unsigned long long to support bigger
range.

Bug: 22461683

Change-Id: I853fe3e8e7dbf0d3e2cc5c6f9688a5a6e1f1fb3e
Signed-off-by: Jin Qian <jinqian@google.com>