2nd pass at optimizing notification listView

* WordPressDB.saveNotes() is now called from a background thread
* Optimized adding multiple notes to note adapter (addAll)
3rd pass at optimizing notification listView

* Note content pre-loaded before being displayed by adapter
* Fixed bug that could cause crash in public Note(Bundle extras) prior to API 12
* Reduced overdraw in notification activity
* Disabling setNotifyOnChange when adding multiple notes (to avoid calls to notifyDataSetChanged for each added item)
Merge branch 'develop' of https://github.com/wordpress-mobile/WordPress-Android into develop