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

tests/unit/src/com/android/settings/notification/app/ChannelListPreferenceControllerTest.java

@@ -91,6 +91,14 @@ public class ChannelListPreferenceControllerTest {
             assertEquals("zeroCategories", mGroupList.getPreference(0).getKey());
         }
 
+        // Test the case with no groups but hidden channels
+        inGroups = new ArrayList<>();
+        mController.mChannelCount = 1;
+        mController.updateFullList(mGroupList, inGroups);
+        {
+            assertEquals(0, mGroupList.getPreferenceCount());
+        }
+
         // Test that adding a group clears the zero category and adds everything
         NotificationChannelGroup inGroup1 = new NotificationChannelGroup("group1", "Group 1");
         inGroup1.addChannel(new NotificationChannel("ch1a", "Channel 1A", IMPORTANCE_DEFAULT));
@@ -250,6 +258,7 @@ public class ChannelListPreferenceControllerTest {
         }
 
         // Test that we go back to the empty state when clearing all groups and channels.
+        mController.mChannelCount = 0;
         inGroups.clear();
         mController.updateFullList(mGroupList, inGroups);
         {