Convert apex mutator to a TransitionMutator

Replace the apex mutator with a TransitionMutator.  Requires moving the
base.apexInfo value into a provider so that it is still present for the
Mutate pass.

Test: go test ./...
Test: no change to out/soong/build-${TARGET_PRODUCT}.ninja
Change-Id: I1c898eaf30b4021f0f96f439cc0b3b3173710fc7