Fix null exception on config change when ScreenDecoration is disabled

Return early on callbacks
  - onConfigurationChanged()
  - onTuningChanged()

Bug: 169438063
Test: 1. disable ScreenDecorations:
         - adb shell setprop debug.disable_screen_decorations true
	 - adb shell pidof com.android.systemui |xargs -i adb shell kill -9 {}
      2. rotate device
Change-Id: Icadfdd0f10e252677392443f3280e5620d0fa797