[iOS] Add blurred background to identity disc when customizing

The identity disc has a number of possible states due to multiple
overlapping ongoing experiments.

First, when the user is signed in, or both the sign in no avatar and ntp
customization experiments are disabled, the old UI is used.

Next, when customization is disabled, there is some old code to preserve
that experiment with no changes.

Finally, when customization is enabled, there are 2 parts to check
separately. The background may be a blur effect or a solid color based
on customization state. And then the foreground styling also varies
based on whether the sign in experiment is enabled.

Screenshots of all the combinations can be found at
https://drive.google.com/drive/folders/1A-VWQ-ABIBiclALP4neDn4YIaQ1SMR-E?resourcekey=0-TQE2XIiaVNqDa9eE5eNEIg&usp=sharing

Bug: 411453982
Change-Id: I04c10b19f1052ab3f032fdc1a38a8c89db0a9371
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6819162
Reviewed-by: Adam Arcaro <adamta@google.com>
Reviewed-by: Pier-Alexandre Bouchard <pabouchard@google.com>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/main@{#1497033}