Don't mutate non-property module fields

Setting sanitize and stl to nil in a mutator isn't valid, if any other
mutator creates a variant then the factory method will be called again
to create the new variants and will reset sanitize and stl to non-nil.
Add a property to sanitize and check it everywhere that checked for
sanitize != nil, and set the Stl property.

Bug: 319288033
Test: all soong tests pass
Flag: EXEMPT refactor
Change-Id: If99d5fa0f088ee4a73cc7dccdab4268618a6009f