Desktop screenshots: Position pre-capture toolbar and capture button

The pre-capture toolbar is positioned at the top center of the display.
Also adds a centered capture button which will take a screenshot when
clicked (fullscreen region only supported currently).

Screenshot: http://screen/ABfhAbsgGZYxq8N.png

Bug: 423967051
Bug: 425440834
Test: Manually verify UI
Flag: com.android.systemui.desktop_screen_capture
Change-Id: I50751f81db7906c698230aa1d324dd35d583f6dc