Disallow scrolling if workspace is in transition.

Because going to overview mode scales down the workspace, it was
thinking the touch was moving even though your finger was still. If
the "movement" was large enough, it was treated as a scroll, causing
jank. This was especially prevalent on tablets due to their size.

Bug: 25779718
Change-Id: Idb7833e0087bd24ca840f6afc451bf221f6bc047