Offer trampoline to help articles.

This allows other parts of the system internals to re-use common
logic for building help links, and it allows all help links to live
in a single common overlay.

Adds a help link slot for "Android is upgrading" notification.

Bug: 30033319, 30122837
Change-Id: I7dec3e63cfb8c7a8ee9a562cc61cb70f3f99fdae