Add permission check to setAllowOnlyVpnForUids

Bug: 63000005
Test: runtest frameworks-net -c com.android.server.connectivity.VpnTest
Test: cts-tradefed run cts-dev -m CtsDevicePolicyManagerTestCases -t com.android.cts.devicepolicy.MixedDeviceOwnerTest#testAlwaysOnVpnLockDown
Change-Id: Ia1a82ee73d8617f3124032986fe6c09c14bf7752