Desktop tests: re-add functional test classes

Many of our test scenario classes now take multiple constructor
parameters, meaning they can't be run as tests (for a test class to be
instantiated by the test runner we need the test class to have a single
constructor with no parameters).

Bug: 364595323
Test: ran tests under scenarios/ and functional/
Flag: com.android.window.flags.enable_desktop_windowing_mode

Change-Id: I79a8310fe3827bef6b2cdf47cbaefe7d3334eb28