New test player stub to load mock native players.

Added a new class TestPlayerStub that takes a magic url in the setDataSource call.
Based on the value of the url, the stub is going to load a DL and create the concrete
player used during the test.
After these initialization steps TestPlayerStub is just a wrapper.

Added a new functional test MediaPlayerInvokeTest to demonstrate how a new
mock player to test the invoke method can be loaded.

Added a new mock player for the invoke test: invoke_mock_media_player.cpp.