Work around problems changing the database journal mode.

Because we always disable WAL mode when a database is first opened
(even if we intend to re-enable it), we can encounter problems if
there is another open connection to the database somewhere.
This can happen for a variety of reasons such as an application opening
the same database in multiple processes at the same time or if there is a
crashing content provider service that the ActivityManager has
removed from its registry but whose process hasn't quite died yet
by the time it is restarted in a new process.

If we don't change the journal mode, nothing really bad happens.
In the worst case, an application that enables WAL might not actually
get it, although it can still use connection pooling.

Bug: 6124556
Change-Id: Ia2ffdbbc8f82721b170f3bf71bd5242dfd56d9ac