Clearing up invalid entries when SyncStorageEngine starts

Any app with permission WRITE_SYNC_SETTINGS could write sync settings
for authorities or accounts that are not valid. This results in invalid
data being persisted to disk which can effectively lead to a DOS style
attack. Clearing such entries on boot will make sure that a reboot fixes
any such issues.

Test: cts-tradefed run cts-dev -m CtsSyncContentHostTestCases

Bug: 35028827
Change-Id: I9e206a42508e3cba65d7523bf47fff743f47dcb2