Set key when Explore preference is created, otherwise tapping it will cause a NPE.
BUG: 25597961

Change-Id: I1e2fc8caa961e1054472b46cbeb6aad407a1f91a
diff --git a/src/com/android/settings/deviceinfo/PrivateVolumeSettings.java b/src/com/android/settings/deviceinfo/PrivateVolumeSettings.java
index 1bfa932..d90d953 100644
--- a/src/com/android/settings/deviceinfo/PrivateVolumeSettings.java
+++ b/src/com/android/settings/deviceinfo/PrivateVolumeSettings.java
@@ -297,6 +297,7 @@
     private Preference buildAction(int titleRes) {
         final Preference pref = new Preference(getPrefContext());
         pref.setTitle(titleRes);
+        pref.setKey(Integer.toString(titleRes));
         return pref;
     }