Test a bug with NETWORK_SETTINGS+registerDefaultNetworkCallback.

Currently, if a process with NETWORK_SETTINGS registers a default
network callback, its uid will be ignored and replaced with an
empty list of UIDs. This means it will incorrectly match VPNs
with any UID range.

Add a test for this bug to make it easier to review the upcoming
change that fixes it.

Bug: 165835257
Test: test-only change
Change-Id: If58524b01fdd60045fb7236d17dedf31fb563f99