Add share intent support (single & multi-file) with proper non-video handling

[NarayanChetri]

Right now there's no way to share a video to NextPlayer from other apps and have it just play. This PR adds that.
The tricky part: a lot of OEM apps (Realme/Oppo's built-in Photos/Gallery being the main one I ran into) don't show "NextPlayer" as an option when you hit share on a video, because they only offer apps that declare a specific video mime type filter, and some of them share with a generic type instead. To get around this, NextPlayer now also registers for the generic / share filter, so it shows up in every app's share sheet, not just the ones that explicitly tag their files as video.
But that creates a new problem: since we're now catching any shared file, people can accidentally (or on purpose) share a PDF, APK, image, etc. to NextPlayer. So I added a check that looks at the mime type of whatever got shared, and if it's not a video, it shows a "not a video file" toast and closes right away instead of opening a blank/broken player screen.

Also handles:
Single file share → plays just that file
Multiple file share (select several videos and share together) → plays them as a playlist, in order
Falls back gracefully if a sender doesn't set a mime type at all (some apps don't), instead of blocking a valid video

Tested on a Realme device (Android 16) sharing from the stock Photos app, a file manager, and WhatsApp.
I went with the / filter + runtime check approach instead of a strict video-only filter since that's what solves the OEM issue this PR is meant to fix. Happy to switch to a stricter video mime type filter instead if that's preferred — Do let me know if this works .

Fixes #1696

[NarayanChetri]

Hi @anilbeesetti , just dropping a quick note to see if there's any feedback on this share intent implementation. Happy to make any necessary adjustments!