[SettingsProvider] Add maxTargetSdk=S to ALWAYS_ON_VPN_LOCKDOWN_WHITELIST

Makes sure that third-party apps that target beyond S cannot access the
setting value directly from Settings.

BUG: 177239688
Test: manual with debug app
Change-Id: I2c5faad7f3e334704addc980530aae84c8c13864
1 file changed