[SettingsProvider] fix connectivity test

TestOnly flag allows the test to read restricted @Readable setting
ALWAYS_ON_VPN_LOCKDOWN_WHITELIST.

BUG: 177239688
Test: atest com.android.server.ConnectivityServiceTest
Change-Id: If41708ad31de9cec3a102b4de4d237e96a448ef1
3 files changed