Add validators for all Settings.Secure settings that are backed up

A few no longer used settings were removed from SETTINGS_TO_BACKUP.

And extend the unit test to fail if new ones are added without
a validator. Also fail to boot in that case.

Ref: go/android-p-backed-up-settings
Test: atest frameworks/base/core/tests/coretests/src/android/provider/SettingsValidatorsTest.java
Bug: 64988620
Change-Id: I94b4039c9f54c341aec72b62579be3dd8bd84dbb