SurfaceComposerClient: Expose default apply token

The default apply token specifies which transaction queue transactions
from a process is sent to in surfaceflinger. This queue is used to
ensure transactions from a process are applied in order.

If a transaction from multiple processes need to be applied in order,
we can use a commit callback to act as a barrier or pass transactions
across processes to be applied on a single process. In some cases
where operations are intertwined it may be advantageous to set a
single transaction queue for multiple processes. In this case
this api can be used to set a single queue to apply transactions by
setting the same apply token on both processes.

Note this api if misused can result in surprising out of order
transactions.

Test: manually set the same applytoken on multiple processes and
verify they go into the same queue.
Bug: 242193885

Change-Id: I507b44686b393c1bb854bd593535a3a3874497af