Transitively add APEX variants for contents of prebuilt_apex/apex_set

This is part of the work needed to allow the exported_java_libs
property to be replaced by exported_bootclasspath_fragments.

Bug: 187266082
Test: m nothing
      m SOONG_CONFIG_art_module_source_build=false nothing
Change-Id: I2c1d70a390200b93163f9799719290c9d55a041c

java/java.go

@@ -1296,9 +1296,10 @@ func (j *Import) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 		if ai.ForPrebuiltApex {
 			if deapexerModule == nil {
 				// This should never happen as a variant for a prebuilt_apex is only created if the
-				// deapxer module has been configured to export the dex implementation jar for this module.
+				// deapexer module has been configured to export the dex implementation jar for this module.
 				ctx.ModuleErrorf("internal error: module %q does not depend on a `deapexer` module for prebuilt_apex %q",
 					j.Name(), ai.ApexVariationName)
+				return
 			}
 
 			// Get the path of the dex implementation jar from the `deapexer` module.