Don't forward apex dependencies to platform when they are added later

IncomingApexTransition normally returns "" for modules that have
no apex variation so that apex variations can depend on non-apex
variations, for example for NDK libraries.

addDependencyOntoApexModulePair uses OtherModuleDependencyVariantExists
to decide whether to add a dependency on a module in an apex.  If
IncomingApexTransition returns "" then OtherModuleDependencyVariantExists
will always return true.  Return the incoming variation when addding
dependencies after the apex transition mutator has already run.

Bug: 319288033
Flag: EXEMPT bugfix
Test: all soong tests pass with later patches applied
Change-Id: Iec40c3be2ed04dca16a9fa6fa0a1c31056b536a8