Avoid reading XML file when it doesn't exist.

The XML file doesn't exist for first time OTA boot or clean device
flash, avoid reading from it.

Bug: b/181346262
Test: adb logcat no longer shows stack trace
Test: atest GameManagerServiceSettingsTests
Change-Id: I833004e9b85a49ce75c24afaeba413da3ca1d969

services/core/java/com/android/server/app/GameManagerSettings.java

@@ -137,6 +137,11 @@ public class GameManagerSettings {
     boolean readPersistentDataLocked() {
         mGameModes.clear();
 
+        if (!mSettingsFile.exists()) {
+            Slog.v(GameManagerService.TAG, "Settings file doesn't exists, skip reading");
+            return false;
+        }
+
         try {
             final FileInputStream str = mSettingsFile.openRead();