Add SettingsValidators class and a test for validators enforcement

Common validators have been moved out to SettingsValidators class,
as they'll be now shared between Settings.System, Settings.Secure
and Settings.Global.

All validators of Settings.System settings have been verified to be
correct and the missing ones were added.

A unit test that verifies that all settings on SETTINGS_TO_BACKUP
list in Settings.System have validators on VALIDATORS list has been
added. This test will also cover Settings.Secure and Settings.Global
once validators are added for those settings. 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: I1fe951604010ab0c3f68a66296885a9766690d69