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

android/apex.go

@@ -610,9 +610,15 @@ func IncomingApexTransition(ctx IncomingTransitionContext, incomingVariation str
 		return ""
 	}
 
-	// If this module has no apex variations the use the platform variation.
 	if len(apexInfos) == 0 {
-		return ""
+		if ctx.IsAddingDependency() {
+			// If this module has no apex variations we can't do any mapping on the incoming variation, just return it
+			// and let the caller get a "missing variant" error.
+			return incomingVariation
+		} else {
+			// If this module has no apex variations the use the platform variation.
+			return ""
+		}
 	}
 
 	// Convert the list of apex infos into from the AllApexInfoProvider into the merged list