Update default user icon to new Material design

This CL introduces a new Material Design default user icon, replacing the previous version.

Key changes:
-   **New Shape:** The default user icon has an updated shape.
-   **Themed Color:** The icon now uses a fixed dynamic color. This means the color is the same for all users but adapts based on the current system theme. Previously, different users had different icon colors.

This feature is guarded by the `aconfig` flag `android.os.material_default_user_icon`. The original icon and color logic will be used if the flag is disabled.

Test: manually on VM. Tested on login, lock screen and settings.
Bug: 424367031
Flag: android.os.material_default_user_icon
Change-Id: Ic7c0ff6254e8e6e786c8a8e07827f7c18ccbb488