Follow-up 36f4daf32c: allow no-write database connections to use transactions

Follow-up 36f4daf32c: allow no-write database connections to use transactions

After 36f4daf32c was merged, Aaron pointed out on Gerrit that:

    Transaction can be used on replicas for REPEATABLE-READ mode, so
    queries in that transaction see the same version of the DB. Those
    do not involve writes and things would probably break sometimes
    without it (like reads following foreign keys and reference in
    separate queries).

So allow no-write connections to call begin() and commit(), and only
throw an exception when a write query is attempted.

Bug: T183265
Change-Id: I138677a714df13ead08061c3e9ef794c3befbe9d