[SettingsProvider] add maxTargetSdk in @Readable

New annotation field that allows Settings key owners to specify the
maximum target sdk level of the caller app allowed to read the
hidden settings key.

Test: atest android.appsecurity.cts.ReadableSettingsFieldsTest
BUG: 187314081
Change-Id: Ibb603df4c8bca75e3788258e69862595c22e5154