refactor(glic): Make window dragging an impl detail

This CL moves the `HandleWindowDragWithOffset` method from the public
`GlicWindowController` interface to the private section of the
`GlicWindowControllerImpl`.

This functionality is an internal implementation detail of how the
window handles dragging and should not be part of the public API.
This change improves encapsulation. The `WindowEventObserver` is
updated accordingly to call the private method.

Bug: b:439904742
Signed-off-by: Bryant Chandler <bryantchandler@chromium.org>
Change-Id: I6a6a696412e7de90ec7adb875e2a66f9f92edf5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6862003
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Trevor Perrier <perrier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1504002}