Only read Themes Stub from system

Make sure themes stub is a system app before parsing

Bug: 137652108
Change-Id: Ibe11f84d1f8bb698711f71308b7398af4d0759dc
diff --git a/src/com/android/customization/model/ResourcesApkProvider.java b/src/com/android/customization/model/ResourcesApkProvider.java
index f770dbc..92e03b9 100644
--- a/src/com/android/customization/model/ResourcesApkProvider.java
+++ b/src/com/android/customization/model/ResourcesApkProvider.java
@@ -25,8 +25,8 @@
             Resources apkResources = null;
             try {
                 PackageManager pm = mContext.getPackageManager();
-                ApplicationInfo stubAppInfo = pm.getApplicationInfo(
-                        mStubPackageName, PackageManager.GET_META_DATA);
+                ApplicationInfo stubAppInfo = pm.getApplicationInfo(mStubPackageName,
+                        PackageManager.GET_META_DATA | PackageManager.MATCH_SYSTEM_ONLY);
                 if (stubAppInfo != null) {
                     apkResources = pm.getResourcesForApplication(stubAppInfo);
                 }