[Android] Get rid of explicit register/unregister in SyncPromoPreference

Replace register and unregister with onAttached and onDetached methods
in SyncPromoPreference

Bug: 1109713
Change-Id: I1bf58fc31e74798a614afdb57179726545ed249d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401522Reviewed-by: Alice Wang <aliceywang@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Tanmoy Mollik <triploblastic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806611}

chrome/android/java/src/org/chromium/chrome/browser/settings/MainSettings.java

@@ -125,7 +125,6 @@ public class MainSettings extends PreferenceFragmentCompat
                 Profile.getLastUsedRegularProfile());
         if (signinManager.isSigninSupported()) {
             signinManager.addSignInStateObserver(this);
-            mSyncPromoPreference.registerForUpdates();
             mSignInPreference.registerForUpdates();
         }
         ProfileSyncService syncService = ProfileSyncService.get();
@@ -141,7 +140,6 @@ public class MainSettings extends PreferenceFragmentCompat
                 Profile.getLastUsedRegularProfile());
         if (signinManager.isSigninSupported()) {
             signinManager.removeSignInStateObserver(this);
-            mSyncPromoPreference.unregisterForUpdates();
             mSignInPreference.unregisterForUpdates();
         }
         ProfileSyncService syncService = ProfileSyncService.get();

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

@@ -69,11 +69,9 @@ public class SyncPromoPreference extends Preference
         setVisible(false);
     }
 
-    /**
-     * Starts listening for updates to the sign-in and sync state.
-     * TODO(https://crbug.com/1109713): Remove this method and use onAttached() instead
-     */
-    public void registerForUpdates() {
+    @Override
+    public void onAttached() {
+        super.onAttached();
         mAccountManagerFacade.addObserver(this);
         mProfileDataCache.addObserver(this);
         FirstRunSignInProcessor.updateSigninManagerFirstRunCheckDone();
@@ -86,12 +84,9 @@ public class SyncPromoPreference extends Preference
         update();
     }
 
-    /**
-     * Stops listening for updates to the sign-in and sync state. Every call to registerForUpdates()
-     * must be matched with a call to this method.
-     * TODO(https://crbug.com/1109713): Remove this method and use onAttached() instead
-     */
-    public void unregisterForUpdates() {
+    @Override
+    public void onDetached() {
+        super.onDetached();
         mAccountManagerFacade.removeObserver(this);
         mProfileDataCache.removeObserver(this);
         AndroidSyncSettings.get().unregisterObserver(this);