Stop requesting a11y focus in GlobalActionsDialog

I'm trying to get rid of hidden API usage and let
TalkBack manage a11y focus itself. TalkBack's
behavior may have changed, so this line was actually
giving focus to the entire screen and TalkBack was
reading the title twice. After removing it, focus
starts on the "Power off" button, and TalkBack reads
first the window title then the button label. I think
that's better.

Bug: 113341835
Test: Brought up dialog with TalkBack.
Change-Id: I9e224ec2e477632781443ca3bde70273179299da