Fix a bug where a spurious capabilities callback is sent.

When a VPN connects and it has any underlying network (which
means almost always, because it will take the default network
if it doesn't declare any), it has default capabilities and
will only take the capabilities of its underlying network
as part of an update happening after making the network
available but before the rematch can take place. This in turn
causes the capabilities callback sent as part of the rematch
to be spuriously sent.

Test: FrameworksNetTests. Also tested together with a
      followup that adds tests with drive-by coverage for this.
Bug: 150570873
Change-Id: Id7d8bba486bada1a7ba5b0f152d2aa02e407f249