Support enabling WAL using a flag when DB is opened.

Using enableWriteAheadLogging() to enable WAL is inefficient because
we previously disabled WAL mode when the database was opened.
Switching from WAL to PERSIST then back to WAL is inefficient
and could slow down application launch time.  It would be better
to leave the database in WAL mode when we open it to begin with.

To do that, we need to know ahead of time whether we will want to
have WAL enabled for the newly opened database.

Using this flag also reduces the chance that we will encounter
an error enabling WAL mode due to there being other open connections
to the database.

Bug: 6124556
Change-Id: I38ec7a528baeda9f1ef77e25e88b3ca4b6296200