Switch `owner` to *string to disable concat

If owner is specified in a defaults module, we shouldn't concat that
with the owner specified in a module using that defaults module. A
string pointer will produce the correct behavior (overriding the default)

Bug: 37330627
Test: out/soong/Android-*.mk looks correct after this change
Change-Id: I64574e2ba81c11b042248d7a44702ec4534ee932
diff --git a/android/androidmk.go b/android/androidmk.go
index b81cc2a..dabefcc 100644
--- a/android/androidmk.go
+++ b/android/androidmk.go
@@ -246,8 +246,8 @@
 		if amod.commonProperties.Vendor {
 			fmt.Fprintln(w, "LOCAL_VENDOR_MODULE := true")
 		}
-		if amod.commonProperties.Owner != "" {
-			fmt.Fprintln(w, "LOCAL_MODULE_OWNER :=", amod.commonProperties.Owner)
+		if amod.commonProperties.Owner != nil {
+			fmt.Fprintln(w, "LOCAL_MODULE_OWNER :=", *amod.commonProperties.Owner)
 		}
 	}