Replace acquire+release thread annotation with excludes (#5944)

[zanderso]

The behavior of acquire+release annotation handling has changed in
https://reviews.llvm.org/D49355 which breaks the build with the new
Clang. However, as has been pointed out, the acquire+release isn't
the right way to prevent double locking as the annotations negate
each other; the correct way is to use excludes or negative requires.
Using excludes annotations also requires using std::lock_guard instead
of std::unique_lock because the latter doesn't have the thread
annotations due to deferred locking which is not needed in Flutter and
so std::lock_guard is a sufficient alternative.