implement active sensing alike

[atsushieno]

Sometimes when a host is killed, the disconnected plugin service spew infinite log lines which means it's running its audio processing forever.

2022-02-16 10:53:41.486 291-2760/? I/chatty: uid=1041(audioserver) HwBinder:291_4 identical 1 line
2022-02-16 10:53:41.499 291-2760/? W/android.hardware.audio.service.ranchu: TinyalsaSink::write:125 pcm_write was late reading frames, dropping 22131 us of audio
...

We should send and receive MIDI active sensing alike i.e. mutual health check messages and shut down if it's dead.

[atsushieno]

related:

• realtime extension controller: support generic bidirectional messaging between host and plugin #73
• isPluginAlive() https://github.com/atsushieno/android-audio-plugin-framework/blob/c5c7484e248c645ce225e065c269c0dfb4da1340/native/androidaudioplugin/core/include/aap/audio-plugin-host.h#L326

[atsushieno]

Copied from #101 :

aap::PluginClientInstance::activate() starts active sensing, and aap::PluginClientInstance::deactivate() stops it. While processing audio, it is supposed to send active sensing messages. It should have no timestamp content as there may be time mismatch.

AAP aap::PluginServiceInstance::updateActiveSensingStatus() is called by the native binder based service implementation, and updates latest_active_sensing_time field (time is managed only within the service process). When aap::PluginServiceInstance::process() runs, it checks the time field and if it was way old (which TBD, but it's 300 msec. in MIDI 1.0 Standard), it does not call the plugin's process() function.

[atsushieno]

I have been rethinking about status updates. In MIDI era we needed active sensing because it is impossible for clients (senders) to find the latest status. For AAP, we can immediately know if AAP Service is not working, as either Binder is shut down, or each operation returns error (we nowadays indeed detect those). Also the services can dump logs for details.

Therefore we do not really need active sensing alike. This can be closed.