UX changes for addons - unsupported item

[Mugurell]

Based on @brampitoyo's comment from mozilla-mobile/fenix#8578 (comment)
and
https://app.abstract.com/share/28d0811b-ffed-4863-89c8-8aaa10acb528?mode=build&selected=3739771863-5D69D8D0-1838-4F55-B7BA-6EE363185D86&sha=e00d4f002ecef58f001903d9111f95b864daa8ae\
we need the following changes:

• use the same style as for the other favicons - Mozac.Widgets.Favicon - icons will have a squircle background.
• ensure the addon icon has 24 dp width/height and uses the same glyph as the other addons icons used in menus, one that has inherent padding (this means it will appear a bit smaller than 24dp).
• ensure the remove icon has 20dp height.
  mozac_ic_delete actually has a 24dp size set but also an innate padding so it can be used as is.

┆Issue is synchronized with this Jira Task

[brampitoyo]

@Mugurell Sorry to change the icon spec one more time. Our add-ons icon should follow the same guidelines as favicons (see mozilla-mobile/fenix#7992).

Unfortunately, in the past, favicons look different on Top Sites, Bookmarks, History and Search Results. Sometimes they have a square background, or round background, or sometimes they have no background at all.

Now that the specs have been unified, we should follow it.

[Mugurell]

To be verified in Fenix - mozilla-mobile/fenix#8578