Load with memory_order_acquire

std::atomic::load operation is with std::memory_order_seq_cst by
default. However, load operation in bufferhubd and buffer hub client
does not need sequentially-consistent ordering which is provided by
std::memory_order_seq_cst. This change changes our load operation with
std::memory_order_acquire so that no reads or writes in the current
thread can be reordered before this load, all writes in other threads
that release the same atomic variable are visible in the current thread.

Test: all tests are still passing.
Test: vega still working.
Bug: 112007999
Bug: 118718713
Change-Id: I2ac75cc306c3de35bf3d953b353f9a9442bdebbc