subtitles: remember category rework

[ynn-dev]

Hi,
I needed a way to enable subtitles for all channel regardless of the channel watched, and since I couldn't find a way in the settings I added it.
I replaced the "Remember enabled subtitles per each channel" switch with a new category called "Remember enabled subtitles" with the following options: "Do not remember", "Remember for each channel", "Remember globally". The logic for the first two options should be the same as before, but the new third option ignores the enabled subtitles channels and shows the subtitles at all times.

It should be almost ready to merge, but there's a couple things that should be kept in mind:

• the category ID in AppDialogUtil.java is at the end of the subtitle IDs, no idea if you want to keep the IDs in order with the UI elements
• same thing with the index for the setting in persistState() inside of PlayerData.java, I added the setting at the end to avoid breaking existing saves
• kept the "subtitle_remember" string in strings.xml as I don't know how to proceed with unused strings. I will be able to add a couple more languages for the new strings in another commit in the future

Thank you for reading and let me know if there are any issues