Package changed/removed listeners for always-on VPN

Fix 2 problems of always-on vpn after always-on package is removed
1. Prevent network being locked down (blocking all network traffic)
   Otherwise, user has no way to download the vpn app from Play Store,
   and never be able to gain control of the network again.
2. Allow user to connect other vpn app.

Implementation
1. Switch off always-on mode if the package gets removed.
2. Restart always-on mode if the package gets replaced/upgraded.

Bug: 29050764
Change-Id: Id3e389ae0b11c6002a5167919292d9634c2014cb