Show toast when untrusted touches happen

Show a toast when untrusted touches happen (either in permissive or
block mode) to help with debugging and get signal from dogfood. The
code will be cleaned before release together with b/169067926.

Test: Enable feature, draw overlay, verify toast appears
Test: atest WindowUntrustedTouchTest WindowInputTests inputflinger_tests
      inputflinger_benchmarks libinput_tests libgui_test
Bug: 158002302
Change-Id: I22da3b24c7b3dd4ae9695827ac00ff29980a6a3c