Delegate fileName into DmfsTask.syncId (#1871)

Signed-off-by: Arnau Mora <[email protected]>

Author: ArnyminerZ

app/src/main/kotlin/at/bitfire/davdroid/resource/LocalTask.kt

@@ -23,7 +23,9 @@ import java.util.Optional
  */
 class LocalTask: DmfsTask, LocalResource {
 
-    override var fileName: String? = null
+    override var fileName: String?
+        get() = syncId
+        set(value) { syncId = value }
 
     /**
      * Note: Schedule-Tag for tasks is not supported