message view / list: fix NPE when list is empty

This happened for example in a starred-message-only view when
un-starting the last message. This led to isFirst() and isLast()
causing a NullPointerException when trying to update the
previous / next buttons.
Add a caching layer to EmailProvider

Database updates can be surprisingly slow. This lead to slow updates of
the user interface which in turn made working with K-9 Mail not as fun
as it should be. This commit hopefully changes that.
Merge pull request #265 from jca02266/master

If there is no message, it should return to a MessageList.
Merge pull request #266 from m0vie/npe-fix

message view / list: fix NPE when list is empty
Fix dialog message when deleting multiple messages from a notification
3.237 changelog
Bumped manifest to 4.327
Update res/raw/changelog.xml
Merge pull request #267 from rk4an/patch-1

Changelog is empty when starting 4.327
Update res/raw/changelog.xml
Bumped manifest to 4.328