Minor tweaks to WifiDetailPreferenceControllerTest.

Add makeNetworkCapabilities and updateNetworkCapabilities
methods, and use them. These are not very useful in the current
code but we introduce in their own CL to limit test changes in an
upcoming CL that will make use them to test new code.

Bug: 62209358
Test: make -j64 RunSettingsRoboTests
Change-Id: I67269e1add40ecb3c2b693548e8bf29ae776a79f