Cancel printing with escape key

When an external keyboard is attached, users expect to be able to cancel
dialogs with the Escape key.  Since PrintSpooler already handles this
behavior when the Back key is pressed, simply give the Escape key the
same behavior.

Bug: 364883486
Flag: EXEMPT bug fix
Test: Manual comparison of ESC and Back on a keyboard with both
Change-Id: I3e54deb1a1cb781fe1d54ea4e19e4476d423e339