Throw an exception if isValidFragment is not overridden for KK targets

Bug: 9901133
Change-Id: I8dfe43aa90546e351f25c94579a34bb87658bf08
diff --git a/core/java/android/preference/PreferenceActivity.java b/core/java/android/preference/PreferenceActivity.java
index 5d886a3..a99705b 100644
--- a/core/java/android/preference/PreferenceActivity.java
+++ b/core/java/android/preference/PreferenceActivity.java
@@ -894,14 +894,11 @@
      */
     protected boolean isValidFragment(String fragmentName) {
         if (getApplicationInfo().targetSdkVersion  >= android.os.Build.VERSION_CODES.KITKAT) {
-            Log.w(TAG, "Subclasses of PreferenceActivity must override isValidFragment(String)"
+            throw new RuntimeException(
+                    "Subclasses of PreferenceActivity must override isValidFragment(String)"
                     + " to verify that the Fragment class is valid! " + this.getClass().getName()
                     + " has not checked if fragment " + fragmentName + " is valid.");
-            // Return true for now, but will eventually return false when all bundled apps
-            // have been modified. TODO: change to return false
-            return true;
         } else {
-            Log.i(TAG, "PreferenceActivity built on pre-KLP launching fragment: " + fragmentName);
             return true;
         }
     }