UsageStatsService: Fix app idle issue at rollover time

App Idle queries are very frequent and so they only check in memory stats.
However, in memory stats can be missing some entries, especially after a rollover, but also
due to a larger bug fixed in master (too risky to take now).

The fix is to do a deep query (reading older files from disk) and maintain a parallel cache
of stats for app idle. That way the rolling window of data required to serve app idle queries
stays in memory.

Bug:26355386
Change-Id: I2dd3946b45d7d893410715bb0534b2b48694ced6