Refactor applyUnderlyingCapabilities and its test.

This reduces verbose assertions and makes the test more compact.
I'm not sure whether it's actually more valuable, since the
current code, while more verbose, is probably more
straightforward to understand.

Also add a test for passing in a null underlying network (i.e.,
follow default network). This requires a minor refactoring in
ConnectivityService because the applyUnderlyingCapabilities does
not currently treat null specially.

Bug: 173331190
Test: test-only change
Change-Id: Ic5a3e16969ea9e1a529706850f148cb0d5fd8e09