[Android] Fix SigninPromo not dismissed after unsetting 'Allow Chrome Sign-in' toggle

Unsetting the 'Allow Chrome Sign-in' toggle should not show the promo
on the main settings page. This bug was created when moving all the
promos to a single class and is due to not registering the class for
SigninAllowedState changes. This cl fixes that issue.

Bug: 1151229
Change-Id: If79c09193bb9e40cbca63770074ea77f5bae22d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2554567Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Commit-Queue: Tanmoy Mollik <triploblastic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830172}

chrome/android/java/src/org/chromium/chrome/browser/sync/settings/SyncPromoPreference.java

@@ -77,7 +77,11 @@ public class SyncPromoPreference
     @Override
     public void onAttached() {
         super.onAttached();
+
         mAccountManagerFacade.addObserver(this);
+        IdentityServicesProvider.get()
+                .getSigninManager(Profile.getLastUsedRegularProfile())
+                .addSignInAllowedObserver(this);
         mProfileDataCache.addObserver(this);
         FirstRunSignInProcessor.updateSigninManagerFirstRunCheckDone();
         AndroidSyncSettings.get().registerObserver(this);
@@ -92,7 +96,11 @@ public class SyncPromoPreference
     @Override
     public void onDetached() {
         super.onDetached();
+
         mAccountManagerFacade.removeObserver(this);
+        IdentityServicesProvider.get()
+                .getSigninManager(Profile.getLastUsedRegularProfile())
+                .removeSignInAllowedObserver(this);
         mProfileDataCache.removeObserver(this);
         AndroidSyncSettings.get().unregisterObserver(this);
         ProfileSyncService syncService = ProfileSyncService.get();