VpnSettings PreferenceList tests

For validating that when VPNs are added / removed, the right set of
changes are made to the PreferenceGroup in which they are supposed to
be shown.

Bug: 30998549
Bug: 29093779
Test: runtest -x packages/apps/Settings/tests/unit/src/com/android/settings/vpn2/PreferenceListTest.java
Change-Id: I9394db0e78cc984ab62e3670aa0a7942ae767a66