Create APEX variation only when needed

Create an APEX variation of a module only when it is either directly
included in an APEX or the dependency to the module demands that the
module should be included. For example, a non-static dependency to a
java library no longer creates an APEX variant of the library.

Bug: 146907857
Test: m (apex_test amended)
Test: cat out/soong/build.ninja | grep "Module:" | wc -l
reduced from 47132 to 45881

Change-Id: Icd79fb4b60668d852b9f018343b934b5e42a8eda

android/apex.go

@@ -185,6 +185,8 @@ func (m *ApexModuleBase) CreateApexVariations(mctx BottomUpMutatorContext) []Mod
 		}
 		variations = append(variations, m.apexVariations...)
 
+		defaultVariation := ""
+		mctx.SetDefaultDependencyVariation(&defaultVariation)
 		modules := mctx.CreateVariations(variations...)
 		for i, m := range modules {
 			if availableForPlatform && i == 0 {