This adds an optional DrmSessionManager#preacquireSession() method
and implements it on DefaultDrmSessionManager.
The manager doesn't promise to keep the preacquired sessions alive, and
will proactively release them if a ResourceBusyException suggests the
device is running out of available sessions in the underlying framework.
In a future change, SampleQueue will preacquire sessions on the loading
thread and keep track of preacquired 'references', releasing them
when seeking or clearing the queue.
Issue: #4133
PiperOrigin-RevId: 358381616
