Refactor SystemUITests module to help create SystemUITests_desktop

This CL creates SystemUITests_defaults, a java default module, by
refactoring the SystemUITests module.

SystemUITests_defaults is used to create SystemUITests_desktop
in ag/35157076

Note that SystemUITests_desktop depends on an overlay located under
google/desktop. Therefore, SystemUITests_desktop cannot be created
under frameworks/base. Otherwise, AOSP builds will be broken, as
reported in b/439667766

Patch set 9 is the reverted CL's change. Compared to the reverted
change, SystemUITests_desktop is not under frameworks/base anymore.

Bug: 438221449
Flag: TEST_ONLY
Change-Id: I126138143d68185826862d30926b9c37ea5cc74f