refactor: improve background streaming handler by optimizing active stream management

- Removed unnecessary clearing of active streams in the Android implementation, allowing for more efficient management of stream IDs.
- Updated iOS implementation to use formUnion and formIntersection for better handling of active and microphone streams, enhancing performance and clarity in stream management.
- Ensured that microphone activation only occurs when there are active microphone streams, improving resource utilization.

android/app/src/main/kotlin/app/cogwheel/conduit/BackgroundStreamingHandler.kt

@@ -266,7 +266,6 @@ class BackgroundStreamingHandler(private val activity: MainActivity) : MethodCal
     }
 
     private fun startBackgroundExecution(streamIds: List<String>, requiresMic: Boolean) {
-        activeStreams.clear()
         activeStreams.addAll(streamIds)
         streamsRequiringMic.retainAll(activeStreams)
         if (requiresMic) {