Android 9 has native support for external cameras, but in order for it to work it should be implemented by the manufacturer, which according to anecdotal reports I saw online isn't done by most or even any of them. I just faced the same problem myself and it seems the best bet is to take an open-source solution and tweak it to support UVC cameras using an ...
Somewhere between Nov 9, 2014 and Apr 17, 2015, MX Player Pro (and also its ad-supported free version) supports both SUB/IDX subtitles.
DVD, DVB, SSA/ASS subtitle tracks.
SubStation Alpha(.ssa/.ass) with full styling.
SAMI(.smi) with Ruby tag support.