Ignore failure when un/registering listener for invalid display

- In the wm tests, the virtual display can be removed by the time we
  go to update the back gesture handler, which can result in an
  exception that (unnecessarily) crashes SysUI.

Bug: 154041177
Test: Manual, not actually able to repro



Change-Id: Iec2a0aaeed43254693a6d2ef01205641189a5648