Added event color (Android) + Updating of calendar color (Android/iOS)

[chris-wolf]

This PR adds loading and updating of event color(Android) and updating calendar color (Android/iOS)

Event color

• Event colors can only be a colors defined by the google calendar ( see retrieveEventColors(Calendar)).
• There is no documentation for event colors so my code is based on the AOSP calendar (https://android.googlesource.com/platform/packages/apps/Calendar.git/+/refs/heads/pie-release/src/com/android/calendar/EventInfoFragment.java)
• Added event color to the example app for testing ( may take couple of minutes till it is updated in the google calendar)
• Other calendars (like local calendar) are unable to have an event color as far as I'm aware
• iOS doesn't support event colors as far as I'm aware so this PR is limited to android
• took the color loading logic from PR Added event color field for Android #516

Calendar color:

• added updating of calendar color for Android and iOS
• google calendar color are updated via colorKey, others are updated using any color

[IVLIVS-III]

Small issue: see inline comment on file example/lib/presentation/pages/calendar_event.dart.

Everything else looks good to me, thanks a lot for your contribution.

[IVLIVS-III]

LGTM. Thank you very much!