Move dexpreopt.Script to android.RuleBuilder

Move dexpreopt.Script to android.RuleBuilder so that the builder
style can be used in more places.  Also add tests for it.

Test: rule_builder_test.go
Change-Id: I92a963bd112bf033b08899e930094b908acfcdfd

dexpreopt/Android.bp

@@ -4,12 +4,12 @@ bootstrap_go_package {
     srcs: [
         "config.go",
         "dexpreopt.go",
-        "script.go",
     ],
     testSrcs: [
         "dexpreopt_test.go",
     ],
     deps: [
         "blueprint-pathtools",
+        "soong-android",
     ],
-}
+}