Introduce dimension of text edit related floating window.

This is a follow up CL of I5a9bcbe29400d6193eb0532a5e711a78a12383cd.
Floating toolbar and suggestion window has a same elevation and
layout_margin. Introduce dimension value for elevation and
layout_margin and refer them.

Bug: 15347319

Change-Id: I6c635398241b60415437707285faeac76a1a4b6d