Add isAlwaysOnVpnPackageSupported to ConnectivityServiceMock

As we add a new @hide method isAlwaysOnVpnPackageSupported to
IConnectivityManager interface, we're adding it to this mock class here
as well.

Test: builds
Bug: 36650087
Change-Id: I787ecfa0a114c6058dd21a48ff00ae3f70d0ce97