Make ImmersiveModeConfirmation trusted

This is the window that appears when an app uses immersive mode for the
first time ("Viewing full screen"), rendered by the system, hence
trusted for go/untrusted-touches.

Fix: 173355690
Bug: 158002302
Test: 1. adb shell settings delete secure immersive_mode_confirmations
      2. Play a video on YouTube, then enter fullscreen, verify clicks
      outside the dialog go through and don't trigger the toast/logcat.
Change-Id: I435a1be99d62579d1ce2eb49129d7b6475a27a7b