Make ScreenshotAnimation window trusted

In order to let touches pass through due to go/untrusted-touches. This
is safe, UI is wholly controlled by sysUI.

Test: 1. adb shell input keyevent KEYCODE_SYSRQ
      2. Touch outside UI, verify no toast or logcat message
Bug: 158002302
Bug: 172717570
Change-Id: Ie322c60876de32926fb4c795459bf8b3f4b0bb11