[auth] Add smaller Activision icon (#6950)

## Description

The current Activision icon is too wide and small to be nicely displayed
in Auth so this PR adds a smaller one, just like the favicon on
Activision's [website](https://activision.com).

I know the Activision icon is pulled from simple-icons and I don't want
to get rid of that, just add an option for a smaller one, but I see that
the smaller Allegro icon is also added but it isn't displayed in the
icon picker and the icon from simple-icons takes precedence so you'd
have to figure this out.

## Tests

I haven't tested this.

mobile/apps/auth/assets/custom-icons/_data/custom-icons.json

@@ -29,6 +29,10 @@
         "certificates.zaka.ai"
       ]
     },
+    {
+      "title": "Activision",
+      "hex": "000000"
+    },
     {
       "title": "Addy.io",
       "slug": "addy_io"

mobile/apps/auth/assets/custom-icons/icons/activision.svg

@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 14.079999923706055 20.530000686645508">
+  <path fill-rule="evenodd" d="m4.71 17.93-.83 2.6H0L7.04 0l7.04 20.52h-3.9l-.81-2.6Zm2.33-7.32L5.9 14.17h2.26z"></path>
+</svg>