Fix UniFile.fromUri() for 2 path segments tree uri

app/src/main/kotlin/com/hippo/ehviewer/ui/settings/DownloadScreen.kt

@@ -94,6 +94,7 @@ fun DownloadScreen(navigator: DestinationsNavigator) {
                                 keepNoMediaFileStatus(downloadLocationState)
                             }
                         }.onFailure {
+                            logcat(it)
                             launchSnackBar(cannotGetDownloadLocation)
                         }
                     }

app/src/main/kotlin/com/hippo/unifile/DocumentsContractApi21.kt

@@ -33,10 +33,15 @@ object DocumentsContractApi21 {
 
     fun createDirectory(self: Uri, displayName: String) = createFile(self, DocumentsContract.Document.MIME_TYPE_DIR, displayName)
 
-    fun prepareTreeUri(treeUri: Uri?): Uri {
+    fun prepareTreeUri(treeUri: Uri): Uri {
+        val documentId = if (treeUri.pathSegments.size == 2) {
+            DocumentsContract.getTreeDocumentId(treeUri)
+        } else {
+            DocumentsContract.getDocumentId(treeUri)
+        }
         return DocumentsContract.buildDocumentUriUsingTree(
             treeUri,
-            DocumentsContract.getDocumentId(treeUri),
+            documentId,
         )
     }