Support %s in product variable properties

Support using strings as product variable substitutions, and
add tests for printfIntoProperty.

Test: varaible_test.go
Change-Id: I06cfadfb1d3fc81da72fb71323706df20426c8b7
diff --git a/Android.bp b/Android.bp
index e2fb864..b46da4e 100644
--- a/Android.bp
+++ b/Android.bp
@@ -72,6 +72,7 @@
         "android/expand_test.go",
         "android/paths_test.go",
         "android/prebuilt_test.go",
+        "android/variable_test.go",
     ],
 }