compose: Support images from keyboard for Android

Fixes: zulip#419
Fixes: zulip#1173

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/widgets/compose_box.dart

@@ -362,6 +362,31 @@ class _ContentInputState extends State<_ContentInput> with WidgetsBindingObserve
     }
   }
 
+  void _handleContentInserted(KeyboardInsertedContent content) async {
+    final url = Uri.tryParse(content.uri);
+    // TODO: is this correct?
+    String filename = (url == null || url.pathSegments.isEmpty)
+        ? 'file'
+        : url.pathSegments.last;
+
+    if (!content.hasData) {
+      // TODO: investigate how data can be null and handle it
+      return;
+    }
+
+    final file = _File(
+        content: Stream.fromIterable([content.data!]),
+        length: content.data!.length,
+        filename: filename,
+        mimeType: content.mimeType);
+
+    await _uploadFiles(
+        context: context,
+        contentController: widget.controller.content,
+        contentFocusNode: widget.controller.contentFocusNode,
+        files: [file]);
+  }
+
   static double maxHeight(BuildContext context) {
     final clampingTextScaler = MediaQuery.textScalerOf(context)
       .clamp(maxScaleFactor: 1.5);
@@ -405,6 +430,8 @@ class _ContentInputState extends State<_ContentInput> with WidgetsBindingObserve
             child: TextField(
               controller: widget.controller.content,
               focusNode: widget.controller.contentFocusNode,
+              contentInsertionConfiguration: ContentInsertionConfiguration(
+                onContentInserted: _handleContentInserted),
               // Let the content show through the `contentPadding` so that
               // our [InsetShadowBox] can fade it smoothly there.
               clipBehavior: Clip.none,