Create frameworks-base-testutils lib

This permits sharing of test components used in connectivity tests.
Rename the MockLooper to TestLooper, reflecting that it is not a
true mock, but a manually controlled Looper for use in tests.

Bug: 28848133
Test: Tests consuming this library pass.

Change-Id: I57e11f9544c7bffdb02739ab2a921512b1b11874