Fix LOCAL_CERTIFICATE of autogenerated RROs

This is case sensitive. lower case `presigned` implies that the apk was signed with a file named `presigned` at the root of the workspace.

Change-Id: I8236f56883b0ca68517e8cade09edb664c3660d9
diff --git a/java/androidmk.go b/java/androidmk.go
index 2ad30b1..0a7178f 100644
--- a/java/androidmk.go
+++ b/java/androidmk.go
@@ -437,7 +437,7 @@
 		Include:    "$(BUILD_SYSTEM)/soong_app_prebuilt.mk",
 		ExtraEntries: []android.AndroidMkExtraEntriesFunc{
 			func(ctx android.AndroidMkExtraEntriesContext, entries *android.AndroidMkEntries) {
-				entries.SetString("LOCAL_CERTIFICATE", "presigned") // The apk will be signed by soong
+				entries.SetString("LOCAL_CERTIFICATE", "PRESIGNED") // The apk will be signed by soong
 			},
 		},
 	}}