Thread safety gaps in AudioStreamAAudio

[philburk]

There are numerous method, eg. getTimestamp(), that reference the underlying native audio stream.
But that stream could get closed by an AAudio error callback that happened simultaneously.

AAudioStream *stream = mAAudioStream.load();
// <-- If close() occurred here then the stream would get used but the pointer is invalid!
if (stream != nullptr) {

We need to lock around references to mAAudioStream.

[philburk]

These may be called from a callback. A lock could hang the join() from the close().
So we need to do a trylock against a special mutex that is only held when closing the stream.
If the try lock fails then return the same error as if the stream was already closed.

[philburk]

A proper solution for real-time audio requires that the get() methods use a std::shared_mutex, which is C++17.