Android: Simplify DeviceFormFactor.isTablet()

Rather than provide a setIsTablet(), just move the resource from
chrome->ui and have DeviceFormFactor use it directly.

Change-Id: Ie867fc604aa223a6c4896124e3dfe80c09675593
Reviewed-on: https://chromium-review.googlesource.com/769348


Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518274}