Move validators out to their own package

Move the settings validators out to their own package so we can
reason about them more easily. This is the first step in a journey
which will end up with us not initialising them inside the Settings
object which, in turn, will reduce boot times.

Bug: 139449903
Test: atest RunBackupFrameworksServicesRoboTests CtsBackupTestCases CtsBackupHostTestCases GtsBackupTestCases GtsBackupHostTestCases
Change-Id: I611d9855fa91bbbbb6b0996e8ed6d765dca74717