Encoder does not reset when disabling a simulcast layer

[graszka22]

To reproduce on iOS:

• have a video track with simulcast with 3 layers with H264 hardware encoding
• disable one layer by setting encoding.isActive to false in peer connection
• start screencast with H264 hardware encoding
• the H264 encoder will fail to init with Failed to create compression session: -12915 error, this is caused by ios encoders limit exceeded (iOS max encoders limitation issue twilio/twilio-video-ios#17 ) however, we should use just 3 encoders (2 simulcast layers + 1 screencast)
• this is probably caused by the encoder not being reset. There is a check here:

  webrtc/video/video_stream_encoder.cc

  Line 131 in a828846

  // No need to reset when stream is inactive.

  that doesn't reset if layer is only disabled and nothing else changed