Fix and improve setting import and export paths

[XiangRongLin]

What is it?

• Bugfix (user facing)

Description of the changes in your PR

• Set ImportExportDataPath only on successful export
• Set ImportExportDataPath only on successful import
• Commit path immediately when import backup

Fixes the following issue(s)

• Fixes Unable to navigate internal storage to export database #6397

APK testing

On the website the APK can be found by going to the "Checks" tab below the title and then on "artifacts" on the right.
https://github.com/TeamNewPipe/NewPipe/suites/2872990922/artifacts/64206682

Due diligence

• I read the contribution guidelines.

[TobiGr]

LGTM
One small thing: please annotate the parameters as @Nonnull

[triallax]

@cubetronic could you please test this PR?

[XiangRongLin]

One small thing: please annotate the parameters as @Nonnull

Done

[Redirion]

could we take the opportunity and replace the System.exit calls with getActivity.finishAffinity() and launch of the main activity?

so

System.exit(0);

to

`getActivity().finishAffinity();
startActivity(getActivity().getPackageManager().getLaunchIntentForPackage(getActivity().getPackageName()));

[XiangRongLin]

could we take the opportunity and replace the System.exit calls with getActivity.finishAffinity() and launch of the main activity?

@Redirion I don't see a reason for why it would NEED to be done here together. Since you already know what those methods do, you could open a seperate PR for that.