Ensure parallel instancing working

[atsushieno]

Apart from #77, it is important to ensure that AAPs can be instantiated multiple times in parallel and can process audio, to prove that it is really useful.

Current aaphostsample and AudioPluginHost are not eligible to achieve this task because only one plugin can be instantiated by them.

helio_workstation and tracktion_engine would be useful, but neither of them works.

[atsushieno]

I believe this is covered by this test which is now applied to various plugins. https://github.com/atsushieno/android-audio-plugin-framework/blob/7ab1dc61d2343bafc3f44d5484d083c2493d7c49/java/androidaudioplugin-testing/src/main/java/org/androidaudioplugin/androidaudioplugin/testing/AudioPluginServiceTesting.kt#L21