No matching variant android_app_import fix

This fixes a bug where android_app_import or android_test_import without
a matching variant or a default apk property value breaks the build.

Test: app_test.go
Fixes: 152343232
Change-Id: I8215aa1a6dad74b503eaa1169a64f8c30dda287c

android/prebuilt.go

@@ -178,6 +178,9 @@ func InitSingleSourcePrebuiltModule(module PrebuiltInterface, srcProps interface
 	srcPropertyName := proptools.PropertyNameForField(srcField)
 
 	srcsSupplier := func(ctx BaseModuleContext) []string {
+		if !ctx.Module().Enabled() {
+			return nil
+		}
 		value := srcPropsValue.FieldByIndex(srcFieldIndex)
 		if value.Kind() == reflect.Ptr {
 			value = value.Elem()