[Ash Notification Drag] Polish drag offset

Right now, the calculation of drag offset does not take notification
image's inside padding into consideration. As a result, when dragging
a small notification image, the drag offset is wrong.

This CL ensures that the drag point is at the top left corner, or top
right corner under RTL.

Bug: b/274459243
Change-Id: Icea708daabfa39a4a040fe770e8b24167828d77c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4348226


Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Andre Le <leandre@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1120047}