SubscriptionController: don't lock sms prompt state

When sending an MMS, subscription infos will be rebuilt, so at one point
we _could_ have 1 sim and then set the prompt enabled to false, then on
the next pass we wouldn't re-enable it, potentially locking the default
SMS sub to whatever sub gets activated first.

Ticket: CYNGNOS-2570

Change-Id: Id5d3d0593f48f2702fc27485905351fbefb970c7
Signed-off-by: Roman Birg <roman@cyngn.com>