Add pref for UserAvatarCustomizationSelectorsEnabled policy

Add kUserAvatarCustomizationSelectorsEnabled pref. In the future, this
will disable profile image download and custom image selection.

Add IsCustomizationSelectorsPolicyEnabled() to fetch new pref value.

  --gtest_filter="*PolicyPrefsTest.PolicyToPrefsMapping*"
  --test_policy_to_pref_mappings_filter="UserAvatarCustomizationSelectorsEnabled"

Bug: b/271606439
Test: out/Default/browser_tests
Change-Id: I66c3bce31d1cb931ca06972084675c981885dfb4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4330376


Commit-Queue: Aiden Chiavatti <chiav@chromium.org>
Reviewed-by: Jana Grill <janagrill@google.com>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118876}