docs: add missing imports to remaining code examples

- android_intent_plus: add import for AndroidIntent usage example
- share_plus: add missing imports to web fallback, XFile.fromData, and URI examples
- include dart:convert import for utf8.encode usage

All code examples now have complete import statements for copy-paste usage.

Author: Udai-Senevirathne

packages/android_intent_plus/README.md

@@ -88,6 +88,8 @@ of integers or strings.
 or get the details of the activity that can handle the intent.
 
 ```dart
+import 'package:android_intent_plus/android_intent.dart';
+
 final intent = AndroidIntent(
       action: 'action_view',
       data: Uri.encodeFull('http://'),

packages/share_plus/share_plus/README.md

@@ -127,6 +127,8 @@ package.
 File downloading fallback mechanism for web can be disabled by setting:
 
 ```dart
+import 'package:share_plus/share_plus.dart';
+
 ShareParams(
   // rest of params
   downloadFallbackEnabled: false,
@@ -140,6 +142,10 @@ You can also share files that you dynamically generate from its data using [`XFi
 To set the name of such files, use the `fileNameOverrides` parameter, otherwise the file name will be a random UUID string.
 
 ```dart
+import 'package:share_plus/share_plus.dart';
+import 'package:cross_file/cross_file.dart';
+import 'dart:convert';
+
 final params = ShareParams(
   files: [XFile.fromData(utf8.encode(text), mimeType: 'text/plain')], 
   fileNameOverrides: ['myfile.txt']
@@ -158,6 +164,8 @@ This special functionality is only properly supported on iOS.
 On other platforms, the URI will be shared as plain text.
 
 ```dart
+import 'package:share_plus/share_plus.dart';
+
 final params = ShareParams(uri: uri);
 
 SharePlus.instance.share(params);