Fix NPE in edge back gesture handler

- TaskBarDelegate creates an EdgeBackGestureHandler, but does not set
  a stage change callback (not needed currently)

Fixes: 197831135
Test: atest SystemUITests

Signed-off-by: Winson Chung <winsonc@google.com>
Change-Id: I35841c077506d7dad35b0c53a72d9907c205cb52