audio: Improve logging in remote submix module

Implement IModule::dump to display the current state in
the audioflinger dump.

Throttle repetitive logging when there is nothing to read.

Remove stale comment for already fixed b/307586684.

Bug: 307586684
Test: adb shell dumpsys media.audio_flinger
Change-Id: I1f1f6e1658d035d46af3a933a825b20a78c7f297