Make default framework location detection on Linux XDG compliant

[Miepee]

As per the XDG basedir spec:

Environment variables: [...] $XDG_DATA_HOME defines the base directory relative to which user-specific data files should be stored. If $XDG_DATA_HOME is either not set or empty, a default equal to $HOME/.local/share should be used.

Implementing this, makes Apktool work in special environments (such as Flatpak) which by default set XDG_DATA_HOME to something else, without the need to override the path with --frame-dir

As far as I know, the BSDs also try to adhere to the XDG spec, so this should be beneficial for them too.

[iBotPeaches]

We do store a few things either temporarily or persisted.

• framework files (persisted)
• temp files (building the apk)
• aapt binaries (temp during building)

So I believe if we are going to respect some environment variables. I might have to do some research how to classify other file storage mechanisms.

[Miepee]

We do store a few things either temporarily or persisted.

Are all of these currently stored in whatever path getFrameworkDir() spews out?

[iBotPeaches]

Are all of these currently stored in whatever path getFrameworkDir() spews out?

Negative. At the base those are like File.createTempFile stuff. https://github.com/iBotPeaches/Apktool/blob/master/brut.j.util/src/main/java/brut/util/OS.java#L131

[Miepee]

Then AFAIK the current approach seems fine.
Only thing that might be worth keeping in mind, is that if Apktool introduces some persistent configuration files, or log files, that those should go into $XDG_CONFIG_HOME / $XDG_STATE_HOME.
Distro maintainers would probably know more about where which files should go on Linux, I'm just some end-user trying to fix a bug that affected me :)