Reland "Phone Hub: Modify Phone Hub tray to show nudge."

This is a reland of commit cfaa8128

Change from original CL: There is now a check to ensure timer does exist before calling to stop it.

Original change's description:
> Phone Hub: Modify Phone Hub tray to show nudge.
>
> Modify Phone Hub tray methods to display Phone Hub nudge if nudge flag is on. Also, hide the nudge when Phone Hub icon is clicked.
>
> UI:
> (Recording of nudge being hidden after clicking on Phone Hub icon)
> https://drive.google.com/file/d/1P9yVCb-aRB7IXY2gqHhorPFXbojnpckz/view?usp=sharing
> https://screenshot.googleplex.com/6DLVFX739n9Us6a
>
> Change-Id: I81ea423963ce395521e749a96b0ac7db33474020
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4163684


> Commit-Queue: Jennifer Serrano <jennserrano@google.com>
> Reviewed-by: Jon Mann <jonmann@chromium.org>
> Reviewed-by: Alex Newcomer <newcomer@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1098164}

Fixed: b/275587468

Change-Id: Ic673e04f3035a58b658806a67f3e31752270787d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4374702


Commit-Queue: Jennifer Serrano <jennserrano@google.com>
Reviewed-by: Jon Mann <jonmann@chromium.org>
Reviewed-by: Pu Shi <pushi@google.com>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1123196}