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
da4a377573