ical4j doesn't need per-thread `contextClassLoader` for `ServiceLoader` anymore

[rfc2822]

See ical4j/ical4j@f89a263

Validate that this is true for all calls in ical4android. Apparently only RuleManager has a ServiceLoader with non-specified class loader in ical4j 3.x, and we don't use RuleManager.

So as I understand it, we can drop all Ical4Android.checkThreadContextClassLoader() and all related checks and comments.

(And to be sure, have a test that parses a calendar with Thread.currentThread().contextClassLoader explicitly set to null)