[Autofill Assistant] Add proactive help to assistant settings

This change moves the Autofill Assistant settings into a separate
screen and adds a new toggle for proactive help.
The Autofill Assistant sub-section is now initially visible
to all users (as opposed to before where it would just appear
after the first use).

Other smaller changes:
 - the link to the settings in the assistant's bottomsheet now
   points to the new page
 - Rejecting the onboarding now leaves the preference unset instead
   of setting it to false.

Bug: b/168178344
Change-Id: I76f419a46598cddb003d95ac8be317c21de72a93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505875
Commit-Queue: Clemens Arbesser <[email protected]>
Reviewed-by: Boris Sazonov <[email protected]>
Cr-Commit-Position: refs/heads/master@{#825856}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 094ff01..832c6bc2 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -5158,6 +5158,12 @@
      flag_descriptions::kAutofillAssistantDirectActionsDescription, kOsAndroid,
      FEATURE_VALUE_TYPE(
          autofill_assistant::features::kAutofillAssistantDirectActions)},
+
+    {"autofill-assistant-proactive-help",
+     flag_descriptions::kAutofillAssistantProactiveHelpName,
+     flag_descriptions::kAutofillAssistantProactiveHelpDescription, kOsAndroid,
+     FEATURE_VALUE_TYPE(
+         autofill_assistant::features::kAutofillAssistantProactiveHelp)},
 #endif  // defined(OS_ANDROID)
 
 #if defined(OS_WIN) || defined(OS_MAC) || defined(OS_CHROMEOS)