Change the variable name "buffer_state_bit" into "client_state_mask".

Reasons:
1. This variable is not refering to a property of the buffer. It refers
to a client of the buffer.
2. The original "buffer_state_bit" of a producer/consumer is actually
the client state bit mask for both buffer_state and fence_state in
shared memory. Thus, "buffer_state_bit" does not make sense for the
fence state.
3. In the future, Every clients takes up two bits in the buffer_state.
For simpler bit manipulation, there will be a future change making the
client_state_bits two bits as well. Please refer to ag/5236978 for an
early look at the future bit manipulation. Thus, this change replaces
"bit" with "mask".

Test: build
Bug: 112007999
Change-Id: I72f59ab9491bd2f135da068f578195fbf5e6c2b6