[Android] Properly validate potentially null InputDevice

As InputDevice.getDevice() makes no guarantees that the returned device
will be non-null, always validate the return value before using in a
meaningful way as a Gamepad device.

BUG=421168

Review URL: https://codereview.chromium.org/644783003

Cr-Commit-Position: refs/heads/master@{#299016}

content/public/android/java/src/org/chromium/content/browser/input/GamepadList.java

@@ -247,6 +247,7 @@ public class GamepadList {
     }
 
     private static boolean isGamepadDevice(InputDevice inputDevice) {
+        if (inputDevice == null) return false;
         return ((inputDevice.getSources() & InputDevice.SOURCE_JOYSTICK) ==
                 InputDevice.SOURCE_JOYSTICK);
     }