To improve functionality of next/prev buttons:
* Stop using managed query for next/prev cursor, so requery won't
run in UI thread. Add explicit lifecycle management.
* Requery on every onResume()
* Use a listener to catch external changes. This applies both to synced
changes and also local deletes.
* If we ever find ourselves out of the list, finish() immediately
to avoid any race conditions.
To reduce flicker & other problems during msg-to-msg navigation:
* Clear attachments list so it doesn't accrete as we navigate
* Clear body so we don't show previous body with newer headers
Fixes bug 2076472 and (possibly) 2069241 & 2078933
6d22ccaa2f