Add possibility to add a backup rules xml file

[tcdude]

This PR enables users to pass a new argument to p4a to enable
custom backup rules through the means of an XML file as documented
in the official Android docs.

I also noticed that the "allowBackup" flag would do nothing when userd
with either webview or service_only bootstraps, where the
AndroidManifest.tmpl.xml would set it to a hardcoded "true" instead of
the user-defined value.

[Jorilx]

This would be very nice to have, thanks!
I had to specify the filename as ../../../../../../backup_rules.xml though, is this expected? Thanks again!

[tcdude]

It is expected, since I duplicated behavior from an existing command line argument. I copied the help text, advising that the path needs to be relative to python-for-android. You can also use absolute paths though, which would make it probably easier to read...

I have too little insight into the inner workings of p4a to implement a better way, but I'm happy to update this PR if someone tells me how to make this relative to the project root or similar.

[tshirtman]

I do think it would be better to make it relative to either the current working directory (os.getcwd()) when the p4a command is called, or from the source directory we tell p4a to package (should be args.private?). using either os.path.join(path, args.backup_rules) or pathlib.Path(path) / args.backup_rules.

[Jorilx]

I have a PR ready to be submitted for adding support for --backup-rules to buildozer :)

[tcdude]

I went for args.private... Looks like I've made a little mess with my latest commit... hope that doesn't create too much trouble to the maintainers when merging this PR...

Sorry

Edit:
Please let me know if (and how...) I can fix the mess I made. I'm happy to force push if necessary, since I really only have a fork to make this PR.

[Jorilx]

I'm not an expert by any means, but maybe it would be easier to just close this PR and open another "clean" one?

[tshirtman]

nothing wrong with force pushing if you rebased/reset the branch to clean up, the PR will update itself if you push to the same branch.

the way i would go about it:

git remote add upstream https://github.com/kivy/python-for-android.git
git fetch upstream
git rebase -i upstream/develop

this will open vim, or whatever EDITOR you have configured.
you want to detele every line that is not a commit by you, in the case of vim, you can do it by typing
V (this will start line selection mode)
} this will jump to the last line of the paragraph, selecting everything
4k this will go up 4 times, so your last 4 commits are not selected, if you need, use j or k to go down/up (well, your keyboard arrows works too, but that's a sin to use them in vim :D), when the selection is of all commits that are not yours press d to delete.

then press :x then enter to save/exit.

Then the rebasing will start, hopefuly it'll complete without issues, if you get conflicts, it's going to be a bit more painful, so let's hit that wall when we get there, if all goes well, just use git push -f to update your branch.

[tcdude]

thanks @tshirtman that seems to have done the trick then :)

Let me know if there are further changes to be included, before this can be merged

[tshirtman]

LGTM