Introduce selected and today states for month popup UI.

This CL introduces selected and today states for months.
These states are similar with the existing ones for day cells.

A new test is introduced to test the new UI: month-picker-appearance-highlight.

Bug: 1003625
Change-Id: Ic4107a6bf8c2d1de14e16db75ea5c8273dd7025a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1808427
Commit-Queue: Ionel Popescu <iopopesc@microsoft.com>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697890}