Mark soong-generated phony rules as PHONY

Test: add --writable=out/ to kati, see fewer warnings with this change
Change-Id: I4beb47f41888e3743b298c90edfeb38fcc0b3d32
diff --git a/java/androidmk.go b/java/androidmk.go
index c6ecfbc..40711f6 100644
--- a/java/androidmk.go
+++ b/java/androidmk.go
@@ -320,7 +320,7 @@
 					fmt.Fprintln(w, "droidcore: checkapi")
 				}
 				if ddoc.updateCurrentApiTimestamp != nil {
-					fmt.Fprintln(w, ".PHONY:", ddoc.Name(), "-update-current-api")
+					fmt.Fprintln(w, ".PHONY:", ddoc.Name()+"-update-current-api")
 					fmt.Fprintln(w, ddoc.Name()+"-update-current-api:",
 						ddoc.updateCurrentApiTimestamp.String())