[FilesBulkPinning] Address UI polish for xf-cloud-panel

This addresses some more feedback relating to the xf-cloud-panel, most
notably:
 1. Drop the panel to popup in line with the gear menu instead of
    slightly overlapping the icon.
 2. Have the button maintain a pressed state when the cloud panel is
    visible and remove when the cloud panel closes.
 3. Update the button background color to be correct for both light and
    dark mode, it was previously correct for light mode by chance, this
    ensures both are correct.

Demo: http://shortn/_15Y5yINVD3

Fixed: b:281607769, b:281608063
Change-Id: I0ab83637520dd0e87263ea34d0969a12574a74ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4514807
Commit-Queue: Ben Reich <benreich@chromium.org>
Reviewed-by: Wenbo Jie <wenbojie@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1141237}