Move active clients mask to an atomic uint64_t in shared memory.

Previously, the active_consumer_bit_mask_ was a private member of producer
channel for creating new consumer buffers with unique buffer_state_bit_
so that newly added consumer can be uniquely identified amoung its
siblings. It was a union of all consumer's buffer_state_bit_.
In the re-design of buffer state, every consumer/producer/client buffers need
to know the location of every other consumer/producer/client in
buffer_state in order to post a buffer: when a client posts a buffer, it
changes its own buffer_state to 00 and others' buffer_state to 10. Thus, it
need to know where the other siblings locates in buffer_state atomic uint64_t.
active_consumer_bit_mask_ suffices this need.
This change moves the active_consumer_bit_mask_ from producer channel to an
uint64_t atomic variable in shared memory, and rename it as
active_clients_bit_mask_ (because it contain both consumers and
producers buffer_state_bit).

Test: marlin-eng on master branch
Test: vega_xr-eng on oc-dr1-daydream-dev branch
Test: buffer_hub-test buffer_hub_queue-test buffer_hub_queue_producer-test
Bug: 112007999
Change-Id: I1ae562701545c7504fd9367c8c8c63a2fd609264