platformSessionId

A platform handle for effects that attach to this device's stream, or null when the platform has no such concept.

Android's audio session id is the only one today, and it is here because nothing above the sink can work it out. LoudnessEnhancer, Equalizer and Visualizer all attach to a session, and a session allocated by an application is one nothing is playing on: the effect attaches and does nothing, with no error. The number has to come from the AudioTrack that is actually playing, which only the sink holds.

Null while no device is open, and null again after close. A released session id still reads like a valid number and effects attached to it are silently inert, so saying nothing is the honest answer and lets a caller wait instead.