Add observable call preview thumbnail in Video sdk (#1619)

* feat: Add observable call thumbnail

* chore: remove usage of `asStateFlow()`

Author: rahul-lohra

stream-video-android-core/api/stream-video-android-core.api

@@ -8748,6 +8748,7 @@ public final class io/getstream/video/android/core/CallState {
 	public final fun getStartsAt ()Lkotlinx/coroutines/flow/StateFlow;
 	public final fun getStats ()Lio/getstream/video/android/core/CallStats;
 	public final fun getTeam ()Lkotlinx/coroutines/flow/StateFlow;
+	public final fun getThumbnail ()Lkotlinx/coroutines/flow/StateFlow;
 	public final fun getTotalParticipants ()Lkotlinx/coroutines/flow/StateFlow;
 	public final fun getTranscribing ()Lkotlinx/coroutines/flow/StateFlow;
 	public final fun getUpdatedAt ()Lkotlinx/coroutines/flow/StateFlow;

stream-video-android-core/src/main/kotlin/io/getstream/video/android/core/CallState.kt

@@ -670,6 +670,9 @@ public class CallState(
         MutableStateFlow<Map<String, Boolean?>>(emptyMap())
     val participantVideoEnabledOverrides = _participantVideoEnabledOverrides.asStateFlow()
 
+    private val _thumbnail = MutableStateFlow<String?>(null)
+    public val thumbnail: StateFlow<String?> = _thumbnail
+
     private var speakingWhileMutedResetJob: Job? = null
     private var autoJoiningCall: Job? = null
     private var ringingTimerJob: Job? = null
@@ -1547,6 +1550,7 @@ public class CallState(
         _settings.value = response.settings
         _transcribing.value = response.transcribing
         _team.value = response.team
+        _thumbnail.value = response.thumbnails?.imageUrl
         didUpdateSession(response.session)
 
         updateRingingState()