Add a common test library.

This is the most common test library for Connectivity tests. It is
meant to be usable in framework tests, network stack tests, CTS,
GTS. To achieve that, it can only depend on framework classes.

Bug: none
Test: NetworkMonitorTest
Test: NsdManagerTest
Test: ConnectivityServiceTest
Test: OffloadControllerTest
Test: NetworkStatsObserversTest
Test: NetworkStatsServiceTest
(all the touched classes)

Change-Id: Ic47cbe7ba0e407145fa6bc49bb2adb3c5937dbc4