Add a system property to disable screen decorations

Provide a way for factory to disable ScreenDecorations to run the Display tests

Bug: 167160540
Test: manual
      1. adb shell setprop debug.disable_screen_decorations true
      2. adb shell pidof com.android.systemui |xargs -i adb shell kill -9 {}
      3. adb shell dumpsys SurfaceFlinger and check if there is no
      screendecoration layers.
Change-Id: I13427b32e7dff22fdef216a6b8a22a9f9926357f