Move `QSFragmentStartableModule` to `ReferenceSystemUIModule`.

`QSFragmentStartableModule` shouldn't be provided in `SystemUIModule`,
as it's not needed in all SysUI variants (in particular, the variants
without shade). `SystemUIModule` should only be used for pieces of
required SystemUI code that variants of SystemUI _must_ include to
function correctly.

Bug: 414597673
Bug: 413766503
Test: Unit tests still pass.
Flag: EXEMPT refactor.
Change-Id: Idca32c344c985ca5bef6041911420dfb8ed0c15b