Fix a bug where the last used times could be wrong.

If a newer UsageStats came along, it would wipe away
the older UsageStats' last used times because it was
newer and, therefore, a source of truth. This is an
improper assumption, however. The newer UsageStat
may cover a timeframe which does not even have a last
use and, thus, destroys valid data.

Bug: 34857041
Test: FrameworksCoreTest
Change-Id: I5111b0d5a184a31a0a8fbdf12984cc7ef90ccd6e