Add support for including app prebuilts in APEX.

Bug: 138429615
Test: Created an APEX containing CtsShimPrebuilt and
CtsShimPrivPrebuilt.
Change-Id: I9d138ccfc03e4b96c9989d2681ad4e4ad5f20108

java/app.go

@@ -985,6 +985,10 @@ func (a *AndroidAppImport) Name() string {
 	return a.prebuilt.Name(a.ModuleBase.Name())
 }
 
+func (a *AndroidAppImport) OutputFile() android.Path {
+	return a.outputFile
+}
+
 var dpiVariantGroupType reflect.Type
 var archVariantGroupType reflect.Type