Glic multi instance: remove GetHost* methods, use GetInstance*

Some external callers will need access to glic UI properties that will
come from GlicInstance and not Host. Callers that need Host can access
it through GlicInstance.

Also start looking up the GlicInstance based on the last acted tab in
GlicActorTaskIconManager.

Bug: b:440969040
Change-Id: I1b2a0c246101f322ad795885542044587425b100
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6951964
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Commit-Queue: Ian Wells <iwells@chromium.org>
Reviewed-by: Alison Gale <agale@chromium.org>
Reviewed-by: Trevor Perrier <perrier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1519057}