debian: Also make /usr/share/waydroid-extra writable

This can be used to easily setup unofficial images for testing e.g. newer Android versions etc. and the path is also mentioned on the Using custom Waydroid images documentation.

For example I've successfully tested a custom Android 11 image under Halium 11 (Volla Phone 22) as working thanks to this :)

I've added this in a way that should also enable this new path to be writable on devices where waydroid was already installed previously, it could be made a bit cleaner if this doesn't matter.

Closes #4 (moved).