aaudio: Load libaaudio.so once globally, rather than for each cubeb context.

[kinetiknz]

Use a global singleton to load libaaudio.so rather than loading (and unloading) it dynamically for each cubeb context.

On Firefox CI or when running GTests locally, a crash with the following stack may occur:

#00 pc 00000000000514e8  /system/lib64/libbinder.so (android::IPCThreadState::joinThreadPool(bool)+564) (BuildId: 6feb4110d56114ae5f35c2b4f7dd713b)
#01 pc 000000000007f398  /system/lib64/libbinder.so (android::PoolThread::threadLoop()+24) (BuildId: 6feb4110d56114ae5f35c2b4f7dd713b)
#02 pc 0000000000019040  /system/lib64/libutils.so (android::Thread::_threadLoop(void*)+248) (BuildId: 97881ed1ac37c033984e97922ff9027b)
#03 pc 000000000001b64c  /system/lib64/libutils.so (libutil_thread_trampoline(void*) (.__uniq.226528677032898775202282855395389835431)+20) (BuildId: 97881ed1ac37c033984e97922ff9027b)
#04 pc 0000000000080df8  /apex/com.android.runtime/lib64/bionic/libc.so (__pthread_start(void*)+232) (BuildId: 2445fc73ede03b9536eef49355aff375)
#05 pc 0000000000073dd8  /apex/com.android.runtime/lib64/bionic/libc.so (__start_thread+64) (BuildId: 2445fc73ede03b9536eef49355aff375)

The crash can happen after the last currently active cubeb context is closed, dependent on recent AAudio activity and scheduling of the various processes involved.

This is a result of unloading libaaudio.so when destroying a cubeb context. libaaudio.so has a global AAudioBinderClient singleton that handles Binder work. If this singleton is destroyed by unloading the DSO while BInder work is in-flight, a crash inside the Binder thread pool will occur.

Since there's no external way to know when it's safe to unload libaaudio.so and it's unnecessary to load and unload a copy for each cubeb context, we can avoid this crash by loading libaaudio.so a single time when first used and only unload it when the singleton is destroyed (as the process exits).

If the crash is still observed when unloading at process exist, we can disable unloading and "leak" the library until process exit. I'll make this change later, if it seems necessary.

[Pehrsons]

Looks solid to me.