Revert name change in OpenGroupPoller

libsession/src/main/java/org/session/libsession/messaging/open_groups/OpenGroupApi.kt

@@ -198,7 +198,7 @@ object OpenGroupApi {
     @JsonNaming(PropertyNamingStrategy.SnakeCaseStrategy::class)
     data class Message(
         val id : Long = 0,
-        val accountId: String = "",
+        val sessionId: String = "",
         val posted: Double = 0.0,
         val edited: Long = 0,
         val seqno: Long = 0,

libsession/src/main/java/org/session/libsession/messaging/sending_receiving/pollers/OpenGroupPoller.kt

@@ -222,7 +222,7 @@ class OpenGroupPoller(private val server: String, private val executorService: S
         handleNewMessages(server, roomToken, additions.map {
             OpenGroupMessage(
                 serverID = it.id,
-                sender = it.accountId,
+                sender = it.sessionId,
                 sentTimestamp = (it.posted * 1000).toLong(),
                 base64EncodedData = it.data,
                 base64EncodedSignature = it.signature,