Refactor Glic screenshot logic to be reusable

This change extracts the screenshot-capturing functionality from
`GlicSidePanelUi` and moves it into the
`BlurredScreenshotViewController`. This makes the logic reusable for
different types of inactive UIs.

Additionally, it refactors the creation of the `GlicInactiveSidePanelUi`
to use two explicit static factory methods: `CreateForVisibleTab` and
`CreateForBackgroundTab`. This clarifies the two different creation
paths and makes the class safer to use by enforcing initialization
rules via a private constructor.

Bug: b:448715457
Change-Id: I6a6a6964f13fc05ede74938d851f449104b3cfcc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7005976
Commit-Queue: Bryant Chandler <bryantchandler@chromium.org>
Reviewed-by: Ian Wells <iwells@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1524295}