Allow runtime selection of audio driver

[igorski]

Currently the engine must be compiled to either use OpenSL or AAudio as the driver. While AAudio promises better performance, OpenSL has a wider range of support.

Additionally, the unit tests use a mocked audio driver which is activated using a compiler directive. This greatly complicates building the library and running unit tests by not being able to do so side-by-side.

Make the engine a runtime option where the DriverAdapter is able to return the requested driver as appropriate. Update the CMakeLists.txt file to create mwengine as a shared library, remove the conditional for RUN_TESTS and make mwengine_unittest link to the shared mwengine. The entry point for unit tests (main.cpp) should auto select the mocked audio driver.

Update the README section on unit tests and "note on .. drivers"

[igorski]

Addressed in 7ab3d10