Fix R8 build errors after updating to ical4j 4.x

[sunkup]

Closes #335

Purpose

Fix R8 build errors after updating to ical4j 4.x.

Short description

This is assuming we don't want to use the optional caffeine caching library that ical4j uses for timezone caching.

The unresolved references come from

• com.github.benmanes.caffeine.** ical4j - Optional caching library. We have timezone cache configured to use MapTimeZoneCache in ical4j.propterties so safe to ignore.
• org.joda.convert.ToString: threeten-extra (@ToString annotation) - I assume that is safe to ignore.
• java.time.zone.ZoneRulesProvider: Runtime support for java.time is provided through desugaring which can lead to a different class path than R8 might expect during static analysis. I think ignoring is safe because DAVx5’s has desugaring enabled.

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[rfc2822]

I'll merge so that I can create a new APK for testing