Added open, dismiss and split screen as custom accessibility actions

In overview with talkback, swiping would select the individual apps
instead of the open and dismiss views. The open and dismiss actions
are changed to accessibility custom actions with the additional action
to split the current app into multi-window mode.

Test: manual - used talkback with running all three actions in
landscape and portrait
Fixes: 29360763
Change-Id: Ie0bec99b849c5754d45577c66b17fa6b0c7e985c