Add a convenience method to update a network's capabilities.

Almost all calls to ConnectivityService#updateCapabilities use
all the current data in the network, and thus call the method
like this:

    updateCapabilities(nai.getCurrentScore(), nai, nai.networkCapabilities);

Introduce a convenience method to simplify this frequent use case.

Bug: 173331190
Test: passes existing ConnectivityService tests
Change-Id: I6eb6d92bd159f2575d10a929bd59f6dd1b7a4b4e