Bug 1243049 - Implement new PrefsHelper native methods; r=snorp

Implement the PrefsHelper native methods. The previous browser.js
implementation supported "pseudo-prefs" that did not exist as actual
prefs, but was accessible through PrefsHelper. In order to accommodate
these pseudo-prefs, we send observer notifications in order to
communicate with browser.js about prefs that we don't support.

widget/android/nsAppShell.cpp

@@ -341,6 +341,11 @@ nsAppShell::Observe(nsISupports* aSubject,
             }
         }
         removeObserver = true;
+
+    } else if (!strcmp(aTopic, "nsPref:changed")) {
+        if (jni::IsAvailable()) {
+            mozilla::PrefsHelper::OnPrefChange(aData);
+        }
     }
 
     if (removeObserver) {