Fix synchronization across callbacks in collective params initialization.

During initialization of local collective params, we may issue RPCs to other
workers in order to obtain device localities.  Currently, we hold a mutex
across these RPCs, but we do not ensure that the thread that unlocks the mutex
is the same as the one that locked it.

This change releases the mutex (InstanceRec::out_mu) before calling
GetDeviceLocalitiesAsync.  Before releasing out_mu, it marks the mutex
unavailable.  Any thread that wishes to acquire out_mu must wait on a condition
variable if the mutex is unavailable.  The callback for
GetDeviceLocalitiesAsync marks the mutex as available again and notifies the
condition variable.

PiperOrigin-RevId: 202346357