Do not intercept Escape with modifiers

When the Escape key is not intercepted, the current behavior will
be to close the system dialogs; however, this will mark it as
handled, and it will never trigger the fallbacks as shown in
Generic.kcm. While fallback shortcuts are flaky and should be
removed, it makes sense that only a Escape without modifiers is
handled as a real Escape.

Bug: 304684672
Test: Flashed on device, confirmed that Escape does not close
system dialogs with modifiers.

Change-Id: Ie19bc382cc1a19ddc7e519c9794318814cfcb83a