Fix: TARGET_FLATTEN_APEX=true does not install any files to /system/apex

94427265d1 broke the flattened APEX
by not updating moduleNames

Test: TARGET_FLATTEN_APEX=true m
Files are under /system/apex

Change-Id: I14b1a6f8b2244d35e1accdf9888cfef65f4b0a03

apex/apex.go

@@ -1046,7 +1046,7 @@ func (a *apexBundle) androidMkForType(apexType apexPackaging) android.AndroidMkD
 		Custom: func(w io.Writer, name, prefix, moduleDir string, data android.AndroidMkData) {
 			moduleNames := []string{}
 			if a.installable() {
-				a.androidMkForFiles(w, name, moduleDir)
+				moduleNames = a.androidMkForFiles(w, name, moduleDir)
 			}
 
 			if a.flattened && apexType.image() {