Clear up classes of lint errors

[nikclayton]

The code contains many instances that Android Lint would warn about.

When linting was made mandatory for CI a "baseline" file was created which grandfathers in those problems so that the lint check will ignore them for the time being (see https://developer.android.com/studio/write/lint#snapshot).

Those issues still need to be cleared up.

This is the top-level issue to do that. If you are interested in working on this:

1. Review https://github.com/pachli/pachli-android/blob/main/app/lint-baseline.xml
2. Identify a class of lint warning you want to fix (e.g., at the time of writing there are many UnknownNullness issues).
3. Create a new GitHub issue to track the work to fix this -- link back to this issue from that one -- so that other people know that you are working on it (the new GitHub "Create sub issue" feature also works well for this).
4. Fix the issue in a new PR

You will need to update the baseline file in the new PR as well. To do that.

1. Recreate the file with ./gradlew newLintBaseline
2. Verify the baseline file deletes the issues you have fixed, and does not add new issues (the line numbers for existing issues may have moved)
3. Add the changes to app/lint-baseline.xml to the PR