OMS: add internal tests

Add unit tests for OMS internal components. These are different from the
OMS public API tests [that reside in core/tests/overlaytest].

Also, fix typo in OverlayInfo#toString.

Bug: 119443459
Test: atest FrameworksServicesTests:com.android.server.om
Change-Id: Icbee70800de5157a1804e0c96101f31c0bad2619