Reduce screen on delay during UsageStats rollover

Decoupled the app idle book-keeping from usage stats lock, by
introducing an mAppIdleLock. This is used for all state related
to app idle. In some cases, the locks will be nested, with
mLock being acquired first and then mAppIdleLock.

This should fix the situation where a rollover, which writes to
disk and could take several seconds when the system is swamped,
like when the device just came out of idle and the screen was
turned on (like this run-on sentence), causes calls from other
services for app-idle status to be blocked. This was resulting
in a long time to turn on the screen.

Also fixed a dump indentation issue.

Bug: 34627115
Bug: 34961340
Test: Manual, force into idle, increased rollover frequency,
      and tested screen on time.

Change-Id: Ie8b44e6f07f82d8a31f1b733a403dd9b6dc310f6