Spam: Close writable DB after use

Writable DBs in the SpamMessageProvider were not being closed due
to the way the methods were chained together.  This patch gets an
instance of a writable SQLiteDatabase and then closes that instance
after performing the required operations.

This patch also removes the moveToFirst call in the MESSAGE query.

Change-Id: Ibe29aaa553b9c1d06cd39bf6c95d6d6f571b9f12