Make MockVpn more realistic and easier to use.

MockVpn is very difficult to use because it requires the test
caller keeping track of both the MockVpn object and an
accompanying TestNetworkAgentWrapper.

It's also not very realistic: for example, connect() doesn't
actually connect anything, it just makes it so that if
ConnectivityService tries to update the capabilities, the attempt
will not be ignored. Also, unlike the real code in Vpn, it
connects with empty NetworkCapabilities (in particular, with
empty UID ranges).

Make this easier to use and a bit more realistic by:
- Allowing TestNetworkAgentWrapper to take a "NetworkCapabilities
  template" that will form the initial capabilities sent when the
  agent registers with ConnectivityService. This allows the VPN
  to register its agent with its UID ranges already set, like the
  production code does.
- Providing separate methods to register the NetworkAgent and
  mark it connected for cases where the test needs to make
  changes to the NetworkAgent before connecting (e.g., poking
  NetworkMonitor).
- Putting the TestNetworkAgentWrapper inside MockVpn and driving
  it through MockVpn's methods. In order not to have too many
  wrapper functions (and because we can't delegate like in
  Kotlin), there's still an agent() method that returns the
  TestNetworkAgentWrapper.

Bug: 173331190
Test: test-only change
Change-Id: I749ff325bc13ac96f512270b86d1f67686eec378