[cts] tests for settings readable fields

This CTS tests that:

1) all public settings keys in Settings.Secure, Settings.System,
Settings.Global that are not marked with @hide must be marked with
@Readable and can be accessed with the `getString` method.

2) some hidden settings keys marked with @Readable can be accessed with
the `getString` method as well.

Test: atest android.appsecurity.cts.ReadableSettingsFieldsTest
BUG: 175024829
Change-Id: Id1f133d52a60f8fb2e938595b84c28ed3a00fe65