Move usage stats data to credential encrypted storage.

Move all of the usage stats data from the regular system storage to
the system CE (credential encrypted) storage.

All events being reported while the user is locked will be stored
in memory and will be processed via the handler after the user is
unlocked. All of these events reported will regularly be persisted to
system DE storage to ensure important events are not lost.
Any query requests will be ignored while the user is locked.

Two new usage events have been added: one to indicate the user has been
unlocked and the other to indicate the user has been stopped.

Also updated documentation for APIs in UsageStatsManager to reflect the
new behavior in R when the user is locked.

Bug: 135484470
Test: atest android.app.usage.UsageStatsTest#testDataIsInCe
Test: atest android.app.usage.cts.UsageStatsTest#testUserUnlockedEventExists
Test: atest android.app.usage.cts.UsageStatsTest
Test: atest UsageStatsDatabaseTest
Change-Id: I18da1a9354b6686472972c5f2304e4c19e28307f