Straighten out database cursors behavior.

Some were never closed, other closed twice. This change
makes all Cursor instances behave, having the #close()
call in a finally{} clause, and puts the burden of closing
the cursor squarely on the creator rather than in the
called methods.
There is however one exception that is beyond the scope
of this change: UserDictionarySettings have a Cursor
member, it's never closed, and fixing the problem is not
obvious. This change adds a TODO for now.

It's not very clear if this change actually helps with
bug#12670151, but it may be related and it's a good
think to do anyway.

Bug: 12670151
Change-Id: I87cc44387e7dee3da1488671b93a28d9d73f7dc0