skins: Move default style modules to getDefaultModules

skins: Move default style modules to getDefaultModules

This advances T140664 as well, because it encourages module
loading logic from the skin to be in this single method.

Update the tests for setupSkinUserCss(), to now target
getDefaultModules() instead, given setupSkinUserCss() no longer
provides these behaviours. Had to move where the mock object
was created so that it can be injected in the skin (previously
it could be passed as parameter). Besides, its generally best-
practice to create mocks and stubs within the test anyhow, not in
the data provider.

Bug: T140664
Depends-On: Ib2b19ba165a9d646a770702cdf1724509156b93e
Change-Id: I3404c1c2a7e8eae0b803b31f333cb9b837f43d4a