Fix glic status icon theming issues on Mac

Previously we made the mac status icon use NSImage's template property
to make the status icon change to be light/dark based on the wallpaper
and transparency setting for the status bar. However because we were
still observing the native theme and updating the image based on changes
to the OS's light/dark mode, after changing light/dark mode the image
would no longer have the template property and thus will no longer
update based on the wallpaper. Since the icon doesn't need to follow
light/dark mode, this CL stops the glic status icon from observing the
native theme on Mac.

Fixed: 393774616
Change-Id: I2361ccd59cc49253675a7661b2960cd714ad90c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6239604
Reviewed-by: Alison Gale <agale@chromium.org>
Commit-Queue: Charles Meng <charlesmeng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1417027}