Use GestureDetector in WorkspaceTouchListener

Replace the current callback logic in WorkspaceTouchListener with the
use of GestureDetector.
That means, platform api is now going to be used for detecting long
press.

There now are some unused variables and unneeded code in this file. It
will be cleaned up in a follow-up CL.

Test: long press on empty home screen
Bug: 126596502
Change-Id: I57302b403f7d7118f2156166d8c707ab3f9ccefd