Remove UserUsageStatsService#userUnlocked.

UsageStatsDatabase#init was being called twice every time - once when
the usage stats service was initialized and once when the user was
unlocked. However, this was unnecessary and doing duplicate work since
on user unlock, when the service is initialized, the database is also
initialized.

Bug: n/a
Test: atest UsageStatsTest
Change-Id: Ic74ba3ff8a0b68c97340ffe25b19065e1a046731