Avoid returning an object that's still used internally

There is no definite path known for this to end up being
touched by other classes, but we could imagine through
some way or some other it ends up shoved in the stringbuilder
pool, leading to catastrophic results.

Hopefully related to
Bug: 5248688

Change-Id: Ib8abfc31263cbf31d515ed607ced5d8253971938