Read embedded title tag instead of always using filename

[trustosas]

Right now the player always shows the filename as the title, even when a file has an actual embedded title tag. Artist tags get read fine, so this seemed like it should work the same way for title.

Turns out the issue is in PlayerActivity.java. The code was force-setting the title to the filename on the MediaItem, which overrides whatever ExoPlayer would've pulled from the file's embedded metadata. Artist wasn't touched anywhere, so it passed through untouched and worked as expected.

This PR:

• Stops forcing the filename into the MediaItem's title/displayTitle unless the title came from an external API caller (in which case that should still win).
• Adds an onMediaMetadataChanged callback so the title bar updates once the file's actual tags are parsed, falling back to the filename if there's no title tag.

Tested on a compiled debug build with files that have title tags and files that don't. Both cases work as expected.

One note: I'm not a Java developer, I worked through the specifics of this fix with AI assistance since I don't read Java. The change is small and I tested it on device, but flagging that up front in case it affects how you want to review it.

[trustosas]

This makes this player the first mainstream player capable of scrobbling music videos properly too.