Create unit test for the VpnInfoPreference

Currently the Settings VPN library doesn't including much testing
support, so this is intended as a start, in order to improve reliability
and stability of the area.

Test: atest -c VpnInfoPreferenceTest
Bug: 187245804
Change-Id: Id5acb699e0a68f479f3b0b6e6614c2e4e43b163b