This CL adds rounded corners and elevation to DropdownPopupWindowImpl.
It also adds a small util function to ApiCompatibilityUtils to
facilitate elevation.

Screenshots: https://docs.google.com/presentation/d/1cSW-l62jY_6HB3UA-KDg8JlguNrVJlncse9HL7xIhuU/edit?usp=sharing

BUG: 874077
Change-Id: Icd01731c2f7292d3ea1397e6790525af00bfa71d
Reviewed-on: https://chromium-review.googlesource.com/c/1265595Reviewed-by: Changwan Ryu <changwan@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598761}