Mark apps as not-idle at least once

On a fresh boot or update to M, mark existing apps
as being used if there is no existing entry in the
usage stats. On subsequent OTAs, make sure that at
least the new system apps are marked as used.

Reduce idle threshold to 1 day.

Bug: 20066058
Change-Id: I9a273c051d04432877bacd381c85bf6e721c1a85