update Host to connect to at most one page handler

This introduces HostManager to create new host instances
for each page handler, instead of allowing Host to manage
multiple page handlers. This is just a step in the right
direction to allow multiple functioning hosts, and simplifying
Host. It's possible to simplify further, but I want to limit
how much change this CL is making.

Bug: b:441352205
Change-Id: I11f667aebfb97d2e671c2e8845c2afe48af1b639
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6892291
Reviewed-by: Trevor Perrier <perrier@chromium.org>
Commit-Queue: Dan Harrington <harringtond@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1508084}