ANDROID: cpufreq: times: optimize proc files

The majority of the time spent reading /proc/uid_time_in_state is due
to seq_printf calls. Use the faster seq_put_* variations instead.

Also skip empty hash buckets in uid_seq_next for a further performance
improvement.

Bug: 111216804
Test: Read /proc/uid_time_in_state and confirm output is sane
Test: Compare read times to confirm performance improvement
Change-Id: I9a735a591b0f1d879dba8e1d9dd05bb47e18a9c5
Signed-off-by: Connor O'Brien <connoro@google.com>