VPN: Move package intent receiver to ConnectivityService.

Currently, PermissionMonitor listen to user add/remove and
package add/remove intent respectively, and so does VPN.
Thus, races might occurr between them.

This commit refactor VPN part by using ConnectivityService to
listen to intents and dispatch events to VPN.

Bug: 118811303
Test: 1. atest FrameworksNetTests
      2. manually add/remove package
      3. cts-tradefed run cts -m CtsHostsideNetworkTests

Change-Id: Id76fd77c5fcfb2b0e21f211f63f007b1ea1aa53f