Adds an option to allow add resources to Android res/ directory

[DexerBR]

Related to kivy/buildozer#1428

Reference: https://developer.android.com/guide/topics/resources/providing-resources

PRs it can cover (at least partially) :
#2330
#2200
#1922
#2299
#2570
#2322

Maybe related issues:
#1964
#2298
#2614

Important notes:

• It is important that the user defined resource folder follows the file structure of the Android documentation linked above.
• If not followed, p4a will perform the copy/overwrite, according to the documentation:
  • Only folders will be copied to res/ directory.
  • Only files will be copied to each of the folders (existing or to be created) in the res/ directory.

Copy/overwrite process steps:

• Clean android resources directory (res)
• Create default/required p4a res directories (I believe this still needs to be defined)

  • Current default res directories:

    • drawable
    • drawable-mdpi
    • drawable-hdpi
    • drawable-xhdpi
    • drawable-xxhdpi
    • layout
    • mipmap
    • mipmap-anydpi-v26
    • values
    • xml

  • Currently in p4a, some of these directories are required by later code, but the directory is not created if it doesn't exist.

  • I'm not sure if they all need to stay or if it would be nice to include others by default (Although they are not needed by p4a at the moment).

• Copy default/required p4a files (currently, many of them are copied on demand along the code. It might be interesting to keep this in one place).
• Finally, copy directories and files (overwrite where applicable) according to user defined folder (via add-res argument) in res folder.

❗Note: Later arguments that change files in the resource folder, such as icon_fg, icon_bg, may override the add-res argument, if the user decides to put an icon in the "custom resource folder", and then pass another file to the icon_fg argument, for example. Changes in this behavior may also need to be evaluated.

Some things need tweaking, but overall, the templating process needs an update.

[misl6]

Superseded by #2684

Thank you!