Update Mobile Data State on subscription changes.

This CL ensures that Vcn instances refresh their mobile data state
when they receive a new TelephonySubscriptionSnapshot. A new snapshot
may contain different subId -> subGroup mappings, which can affect
whether a Vcn instance's subGroup has mobile data enabled or not. This
in turn can change which VcnGatewayConnections are brought up by the
Vcn.

Bug: 191394092
Test: atest FrameworksVcnTests CtsVcnTestCases
Change-Id: Ib59b4ed7900a177b17ed4a6e197e271b10937f65