Switch dex_bootjars to a singleton module

The main difference between a singleton and a singleton module that
affects this change is that the latter requires a module to be defined
in order for the singleton to be instantiated. This change defines the
module in both the tests and the standard build.

Bug: 177892522
Test: m droid
Change-Id: Id6669835cf3a2ab7359fa53047eb9042992a9362
diff --git a/java/testing.go b/java/testing.go
index 0327963..0b1e2eb 100644
--- a/java/testing.go
+++ b/java/testing.go
@@ -204,6 +204,13 @@
 		`, extra)
 	}
 
+	// Make sure that the dex_bootjars singleton module is instantiated for the tests.
+	bp += `
+		dex_bootjars {
+			name: "dex_bootjars",
+		}
+`
+
 	return bp
 }