Fix the legacy VPN tests that start racoon.

These tests do not actually work, or at least not as designed.
What happens when they are run is that creating/connecting the
VPN throws an exception. The call to legacyRunnerReady.block()
times out after 10 seconds because the condition variable is
never opened, but the doesn't notice the timeout becasue it
doesn't check the return value of block().

There are two reasons for the VPN not coming up.
1. VpnConfig.getIntentForStatusPanel calls into PendingIntent and
   ActivityManager statics, which bypass all the mocks and fail
   with an appops denial. Fix this by making it a dependency.
2. The tests are broken due to the UserManager API changes
   tracked in b/175883995. Fixing by adding a bit of ad-hoc code
   into startLegacyVpn, with a TODO to delete it once the rest of
   the UserManager setup code is fixed.

Fix these and check the return value of block(). This ensures
that if any other breakage is added the test will actually fail.
Also check that the throw route survives all the way to the
LinkProperties sent to the agent.

Bug: 173331190
Test: atest com.android.server.connectivity.VpnTest
Change-Id: Ieb7f33bce283ac5ee562a912df8edb9c930ed2b0