[mob][photos] Use media_kit player for videos not supported by native…

… player (#4429)

## Description

On iOS, if a video is unsupported by the native player, the app will
switch automatically to use media_kit player.
On android, user will have to manually switch to the media_kit player.  

I used white with 20% opacity for the icon that switches to the MediaKit
player, making it most noticeable against the darkest background (black,
when the video isn't playable) and less prominent against lighter
backgrounds.



https://github.com/user-attachments/assets/b4316c6e-1691-4328-8984-4d8240179873



Have fixed a UI/UX issue on the video player seen when the video has a
caption/description.

mobile/lib/events/use_media_kit_for_video.dart

@@ -0,0 +1,3 @@
+import "package:photos/events/event.dart";
+
+class UseMediaKitForVideo extends Event {}