Take capability below API 24, and add comment.

Change-Id: I86d99fa9e74a8475c0b2bad202cfb4697ab1016b

core/network/src/main/kotlin/com/google/samples/apps/nowinandroid/core/network/demo/DemoNiaNetworkDataSource.kt

@@ -51,7 +51,10 @@ class DemoNiaNetworkDataSource @Inject constructor(
             if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
                 assets.open(TOPICS_ASSET).use(networkJson::decodeFromStream)
             } else {
-                networkJson.decodeFromString(assets.open(TOPICS_ASSET).toString())
+                // Use decodeFromString to capability with API 24 below.
+                // https://github.com/Kotlin/kotlinx.serialization/issues/2457#issuecomment-1786923342
+                val topicsJsonString = convertStreamToString(assets.open(TOPICS_ASSET))
+                networkJson.decodeFromString(topicsJsonString)
             }
         }
 
@@ -61,7 +64,10 @@ class DemoNiaNetworkDataSource @Inject constructor(
             if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
                 assets.open(NEWS_ASSET).use(networkJson::decodeFromStream)
             } else {
-                networkJson.decodeFromString(assets.open(TOPICS_ASSET).toString())
+                // Use decodeFromString to capability with API 24 below.
+                // https://github.com/Kotlin/kotlinx.serialization/issues/2457#issuecomment-1786923342
+                val newsJsonString = convertStreamToString(assets.open(NEWS_ASSET))
+                networkJson.decodeFromString(newsJsonString)
             }
         }