Better error handling in UsageStats.

Ensure all types of exceptions are caught and logged/handled within
UsageStats.

Also update how read failures for pending events files are handled -
since the entire System DE directory is deleted on a successful user
unlock, read all the pending events that you can and skip over
potentially corrupted files.

Bug: 145574371
Test: atest UsageStatsDatabaseTest
Test: atest android.app.usage.cts.UsageStatsTest
Change-Id: I2e22fb01f9b97961b2160aeebaf7791d6673f8b9