Switch genrule-phony to ninja phony rule

Instead of using android.Phony, which is a "real" rule, use
blueprint.Phony, which is ninja's built-in "phony" rule. That's desired
in this case, since we only want this to be an alias to the inputs.

Test: cd system/bt; mma; mma (second run had nothing to do)
Test: treehugger
Change-Id: Ib0b630e23653c05a12cf617350f8b4c6d6d6cdff
diff --git a/genrule/genrule.go b/genrule/genrule.go
index cf0b484..b8b0e01 100644
--- a/genrule/genrule.go
+++ b/genrule/genrule.go
@@ -437,7 +437,7 @@
 		phonyFile := android.PathForModuleGen(ctx, "genrule-phony")
 
 		ctx.Build(pctx, android.BuildParams{
-			Rule:   android.Phony,
+			Rule:   blueprint.Phony,
 			Output: phonyFile,
 			Inputs: g.outputFiles,
 		})