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

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: Ibd4a2bad0c6a1f2a9e1beec1a4ec8e6972fd86a4