Bug 1073328 - Prevent using our own handler as system handler; r=snorp

mobile/android/base/GeckoAppShell.java

@@ -212,7 +212,9 @@ public class GeckoAppShell
     public static native void dispatchMemoryPressure();
 
     public static void registerGlobalExceptionHandler() {
+        if (systemUncaughtHandler == null) {
             systemUncaughtHandler = Thread.getDefaultUncaughtExceptionHandler();
+        }
 
         Thread.setDefaultUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
             @Override