Move MutateApexTransition shortcut later

Returning from MutateApexTransition too early is allowing invalid
apex_available values to be ignored for modules that have no apex
variations.  Move the exit later so that apex_available checks
happen and platform variants are hidden consistently.

Bug: 394664568
Test: builds
Change-Id: I324de8090a9f1a874b5b22f20b8d62bf22c7d79c