Process Google Calendar-private extended property "iCalUid"

[rfc2822]

When importing an iCalendar with a given UID, Google Calendar doesn't set the UID_2445 field, but instead generates an ExtendedProperty named iCalUid that contains the UID.

ical4android/DAVx⁵ currently doesn't process this ExtendedProperty and thus ignores the UID. We could instead take the UID from iCalUid when UID_2445 == null.

Depends on bitfireAT/davx5-ose#538
Depends on #128
Useful for https://github.com/bitfireAT/davx5/issues/511

Depends on #130

[github-actions]

This PR/issue depends on:

• bitfireAT/davx5-ose#538
• #128
• #130