Add base classes for common ConnectivityService tests.

This sets up what is necessary for an instrumented
ConnectivityService to run. Users of this class are
meant to inherit CSTest.

This is still relatively basic and does not have all the
instrumentation in ConnectivityServiceTest. Developers
looking to extend CSTest may find some instrumentation
missing ; when they add the missing instrumentation,
they should consider whether it should be generic for all
CSTests (and put it in base/), or whether it's local to
their own test suite. This should enable faster testing
as each CSTest children will only need to set up the
instrumentation it actually needs.

This patch also migrates a basic test to have a first user.

Bug: 272685721
Test: ConnectivityServiceTest
      CSBasicMethodsTest
Change-Id: I1c47f616af90629c9cb2a6ae89d992b19863e704