ANDROID: cpufreq: stats: Fix dead stats clearing timing

Combine task exit stats aggregation to happen at the same
time when stats are cleared, avoiding inconsistent data
(stats of uids time appear to decrease) and simplifying
the code.

Bug: 63041570
Test: Continuosly read /proc/uid_time_in_state and check
for decreasing values

Signed-off-by: Andres Oportus <andresoportus@google.com>
Signed-off-by: Connor O'Brien <connoro@google.com>
Change-Id: Ie2b4ce4a02f533140a6c5f9ae5cb7ae978c93f9e