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