pw_sync: Update ScopedLocker to support std::adopt_lock

This is useful in conjunection with Lockable.try_lock(), e.g.:

  pw::Status Example(bool may_block) {
    if (may_block) {
      mutex_.lock();
    } else if (!mutex_.try_lock()) {
      return pw::Status::Unavailable();
    }
    pw::ScopedLocker lock(mutex_, std::adopt_lock);

    // ...
  }

Change-Id: I6e9c43ac18d9877917fd83379b6b3067a791806d
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/446252