UserManager restricted profile SystemApis

Makes two new SystemApis:
getRestrictedProfileParent()
canHaveRestrictedProfile()

Temporarily disables VPN Tests that rely on the old APIs until those
tests are updated (b/175883995).

Bug: 171529940
Test: atest FrameworksNetTests:com.android.server.connectivity.VpnTest
Test: Tests for UserManager SystemApis are TODO awaiting completion of new user test infrastructure (b/163890431)
Change-Id: I28e39400039631e7d391dc7b0d003e8a38d1f06a