Previously, on Android devices, when the gr-reply-dialog overlay is
opene, important contents could get cut off, including the submit and
cancel buttons. This change makes the reply dialog scrollable and full
width on small screens. The max-height is also 100% so if scrolling is
required, the overlay will appear full screen.
Bug: Issue 4944
Change-Id: Ibf265a3039129363e479cbb3c29cb887aa23d889