feat(theming): Implement Material 3 Theme

[stefan-niedermann]

This PR aims to align the theming between Files, Talk, Notes, Deck and News. (see also: nextcloud/notes-android#1752)

The mechanics are a bit different, because News does not support fetching the account color (yet) - however this makes it a bit easier, because we only have to define a static theme instead of calculating and applying it at runtime to each and every UI component.

_	_

Remaining issues:

• ⚠️ When setting theme manually to dark / light, the statusbar and NewsReaderListFragment have inconsistent state compared to the rest of the app
  I can only assume that it has to do with Dependency Injection or some other magic, but calling Splashscreen.installSplashScreen(this) lead to the broken behavior. Removing it solved the issue (tested on API 33 and API 28)
• Popup menus do have strange partial background colors at the top and bottom
• NavigationDrawer feed items do have strange background
• Merge styles.xml and styles-v21.xml, because the minSdk is already 21
• Implement proper Splashscreen
• Export / Import OOPML seem to be not working - I doubt that this is related to the theming changes, but it's worth a look
• ⚠️ Podcast-stuff needs to be checked (I will need some help here because I don't use this)
• ⚠️ Menu icon tints are not correct yet - needs some checking
• Shape of the favorite action drop menu is still circular - needs adjustment for new FAB shape?
• Favorite actions are bitmap graphics - Keeping them @color/nextcloudBlue because I don't know how they have been generated
• Check whether Black OLED Theme still works as expected everywhere
• Manual Login screen is not looking quite right
• Feed dialogs are not in Material 3 theme

[Unpublished]

Export / Import OOPML seem to be not working - I doubt that this is related to the theming changes, but it's worth a look

Should be #1188

[stefan-niedermann]

Woohoo 🥳 I didn't notice you picked up the work on this PR again @David-Development !

Thanks for finishing & merging!

[David-Development]

@stefan-niedermann Well.. you did most of the work so thank you for getting the ball rolling! 🥳