Move markPlatformAvailability mutator before apexMutator

This will allow the apexMutator to use the NotAvailableForPlatform()
method initialized by the markPlatformAvailability mutator to help
determine whether to create a platform variant or not.

Bug: 187910671
Test: m droid
Change-Id: I745b4428afc68e7ca4cd2f4cbe810b8df1eebdb7