Reland: Basic Pagination for System Tray

Add logic to set FeaturePodsContainer Layout according to page numbers
Add PageIndicatorView in UnifiedSystemTrayView

The SystemTray will be limited to show 3x3 FeaturePodButtons at a time.
Additional Buttons will be added to additional pages.
The available pages will be signified by small buttons under the
feature pods container.

Bug: 914077
Change-Id: I03fda5df5e615216d51ca371f2386a69cf6b1049
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1613565
Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Reviewed-by: Tim Song <tengs@chromium.org>
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660497}