Sharesheet - Support landscape mode

Measure the current icon width, and support a max of 4 icons wide in
portrait mode, or 8 in lanscape mode. Also correct an issue with cell
visibility; app targets were fading out when they should've been
instantly invisible.

Bug: 120419081
Test: atest ChooserActivityTest and visual inspection
Change-Id: I635532aff362b65900ec02bafaae818267b2d51f