alsa: fix possible threading issue

An std::thread, ctor constructer, must be the last members in a class to be sure
that all previous members are initialized in class ctor before to run the thread
itself.
This may fixes random issues seen with others member accesses.

Change-Id: I7ceb0cf522236411f4a2faa34ff4f32eae0e4c13
Tuleap: #171