Make java_test_helper_library installable

java_test_helper_library should product an installabler, dexed
library, the same as java_test just without the auto-generated
test config.

Test: m checkbuild
Change-Id: Ie573c1d24969ba9c4feb8a2e85f8969d5d713064
diff --git a/java/java.go b/java/java.go
index d6c759b..fac28ed 100644
--- a/java/java.go
+++ b/java/java.go
@@ -1632,6 +1632,9 @@
 		&module.Module.protoProperties,
 		&module.testHelperLibraryProperties)
 
+	module.Module.properties.Installable = proptools.BoolPtr(true)
+	module.Module.dexpreopter.isTest = true
+
 	InitJavaModule(module, android.HostAndDeviceSupported)
 	return module
 }