Let notification shade control system bar visibility

When immersive mode confirmation dialog is shown, we choose notfication
shade as the system bar controlling window as long as it can receive key
events.

Fix: 241872473
Test: 1. Show immersive mode confirmation dialog in landscape mode.
      2. Enter AOD mode without entering lockscreen mode.
Change-Id: I2b9fb12f051bef47cbd21786c6d22e86c1351879