ANDROID: uid_sys_stats: report uid_cputime stats in microseconds

A porting error in commit 0ae80885 ("ANDROID: uid_cputime: Adds
accounting for the cputimes per uid.") caused
/proc/uid_cputime/show_uid_stat to report times in milliseconds
instead of microseconds. Restore the expected behavior.

Bug: 133273154
Signed-off-by: Connor O'Brien <connoro@google.com>
Change-Id: I203919d1d44b300dab91810b16626a6950b063f8