Draft: VPN Service communication

@ankitgusai19 Let's discuss this PR. I've separated your commit from !19 (closed), since these changes are unreleated.

-ArtiVpnService is now bound to HomeFragment and AppListingFragment(created AbsVpnServiceFragment to isolate service connection logic) -Removed static VpnStatusObservable as ArtiVpnService now exposes same functionality -Some fine-tuning

(cherry picked from commit 251aee6a)