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

Android.bp

@@ -222,3 +222,8 @@ cc_genrule {
     srcs: [":linker"],
     out: ["linker.flags"],
 }
+
+// Instantiate the dex_bootjars singleton module.
+dex_bootjars {
+    name: "dex_bootjars",
+}