telephony: Improve compatibility for older DSDS stacks

Older stacks require that you update the preferred network when
you are changing data subscriptions.  The update to the network mode
internally triggers changes between a 2G only stack and a full
stack.

To support this always set the preferred network mode when changing
data subscriptions.

Additionally, ensure that the full stack is released before attempting
to bind the new subscription to that stack rather than seting them
concurrently.  When set concurrently,  there will be a race condition
as both attempt to use the full stack.

Instead, switch to the 2G stack and only after that completes
make the second switch.

Change-Id: I7de8ac803081362f6fbfce29d9578a3eb5d10c30