SF: Cleanups to use std::atomic/std::mutex

A few places in the code used "volatile". This patch removes the
volatile.

In most of the cases, the volatile value was converted to a std::atomic
value. The exception was Barrier where it was not actually necessary,
but there I converted the code to use std::mutex.

Bug: None
Test: atest SurfaceFlinger_test CtsViewTestCases
Change-Id: I7b4f5a7398a241db8201dc7f60d7c9cd41e32e1b