Add support for the BraveAIChatEnabled group admin policy in iOS

Test Case

• Visit brave://policy/test
• Add the Brave AI Chat Enabled policy (and choose to Disable it)
• Tap "Apply Policies"
• Verify AI Chat/Leo is not accessible while the policy is active:
  • Leo removed from the browser menu (•••)
  • Leo removed from Settings
  • Leo quick search engine removed from search engine bar while typing in the URL bar
  • Known gap: Widget shortcut options are hard-coded in the intent definition, so we can't currently hide the option but verify opening Leo from shortcuts widgets does nothing