Make android_test_helper_app support apex_available

Needed in order to enforce apex_available is set for
everything that gets added to apexes.

Bug: 147364041
Test: apex_test.go
Change-Id: I083ec35283c4c7b5101d00139416d0734b97bab4
diff --git a/java/app.go b/java/app.go
index 05fa505..a6b3408 100755
--- a/java/app.go
+++ b/java/app.go
@@ -752,6 +752,7 @@
 
 	android.InitAndroidMultiTargetsArchModule(module, android.DeviceSupported, android.MultilibCommon)
 	android.InitDefaultableModule(module)
+	android.InitApexModule(module)
 	return module
 }