Fix `image_picker` for Android >= Q

Per https://pub.dev/packages/image_picker, the image picker plugin
needs legacy external storage permission on Android Q and above
for now; it does not yet support the new permission model.

Fixes #306.

Signed-off-by: Michel Alexandre Salim <michel@michel-slm.name>

android/app/src/main/AndroidManifest.xml

@@ -13,7 +13,8 @@
     <application
         android:name=".Application"
         android:label="FluffyChat"
-        android:icon="@mipmap/ic_launcher">
+        android:icon="@mipmap/ic_launcher"
+        android:requestLegacyExternalStorage="true">
         <activity
             android:name=".MainActivity"
             android:launchMode="singleTask"