Avoid CursorIndexOutOfBoundsException when deleting a message

Deleting a message creates an entry in EmailProviderCache so
EmailProviderCacheCursor can skip the Cursor row during the time
it takes to update the database.
Previously EmailProviderCacheCursor.isLast() returned the wrong
result when the last row in the wrapped Cursor was hidden. This
lead to the crash in MergeCursor.

Fixes issue 5820