Layout improvements for notification pages

Changes for upcoming theming changes in Settings:
- replacing some margins with padding
- merging some (switch pref + info pref) combos into a single preference.
I flagged the one that was already launched, but changed the other directly
- added some section headers (unflagged)
- moved all app wide notification settings to a single section (unflagged)
- changed two plain text prefs into TopIntroPreference, the dedicated pref
type for that sort of UI
- fixed some UI issues with 'Show more categories' appearing too often
- removed a duplicate notifcation channel label (unflagged)
- replaced a button layout preference with ButtonPreference (unflagged)

Test: manual review with is_expressive_design_enabled on and off
Test: atest com.android.settings.notification.app
Flag: EXEMPT this feature is not using aconfig for flagging
Bug: 349652992
Change-Id: I2acd7b2eb9dbcf6929143bfde99cd67163f1f95d

src/com/android/settings/notification/app/AllConversationsPreferenceController.java

@@ -23,6 +23,7 @@ import androidx.preference.Preference;
 
 import com.android.settings.R;
 import com.android.settings.notification.NotificationBackend;
+import com.android.settingslib.widget.TopIntroPreference;
 
 public class AllConversationsPreferenceController extends ConversationListPreferenceController {
 
@@ -39,9 +40,9 @@ public class AllConversationsPreferenceController extends ConversationListPrefer
 
     @Override
     Preference getSummaryPreference() {
-        Preference pref = new Preference(mContext);
+        Preference pref = new TopIntroPreference(mContext);
         pref.setOrder(1);
-        pref.setSummary(R.string.other_conversations_summary);
+        pref.setTitle(R.string.other_conversations_summary);
         pref.setSelectable(false);
         return pref;
     }