Prepare for a type-safe OnceKey

Add an opaque OnceKey type and use it for all calls to Once in
build/soong.  A future patch will convert the arguments to
Once* to OnceKey once users outside build/soong have been updated.

Test: onceper_test.go
Change-Id: Ifcb338e6e603e804e507203c9508d30ffb2df966
diff --git a/java/android_resources.go b/java/android_resources.go
index efd3e3d..44cb709 100644
--- a/java/android_resources.go
+++ b/java/android_resources.go
@@ -46,7 +46,7 @@
 	paths android.DirectorySortedPaths
 }
 
-const overlayDataKey = "overlayDataKey"
+var overlayDataKey = android.NewOnceKey("overlayDataKey")
 
 type globbedResourceDir struct {
 	dir   android.Path