Use isAdminRoute for admin page check.

Problem/Motivation

Currently the module is checking the start path of the url being accessed for "/admin" to determine if the page you're hitting is an admin page. That check only matches some of the pages though, as it ignores /node/*/edit, /batch, and other 'admin' pages.

Proposed resolution

Replace the path check with the AdminContext isAdminRoute check, which handles that logic for us and is much more comprehensive. That's what this patch does.