Fill ConnectivityService in testDumpDoesNotCrash()

testDumpDoesNotCrash does not dump enough objects to have full
coverage. e.g. In b/303348620, it did not detect the crash since
there is no NetworkAgentInfo stored in ConnectivityService.

This change ensures ConnectivityService is filled with more
objects that will be dumped:
1. NetworkProvider
2. NetworkAgentInfo
3. NetworkOffer
4. Network preferences

Bug: 303348620
Test: atest FrameworksNetTests
Change-Id: I7e2f1954c91409f3bf0daf01300e34ce73e311cb