Align empty recents message+icon to center of View.

Seems like it was previously aligned to center of task which doesn't always look good.

Screenshots:
Handheld before - https://screenshot.googleplex.com/6GmqhTjUhUpwwBs
Handheld after - https://screenshot.googleplex.com/3QMYCQaS6USMQYy

Tablet before - https://screenshot.googleplex.com/7tsERFGVsa6QSNY
Tablet after - https://screenshot.googleplex.com/8itDgQ2jDTHk463

Unfolded before - https://screenshot.googleplex.com/AJaqC9LqP2Dwfz2
Unfolded after - https://screenshot.googleplex.com/8aaa7ZhemqHXHRh

Fix: 299100892
Test: OverviewEmptyMessageImageTest and observed on tablet, handheld and unfolded.
Change-Id: I538d25e5a98e25351e71718de3ca572fd00438ad