EditCalendarActvivity: Switch to compose state and kotlin flows

[sunkup]

Switch to compose state and kotlin flows

• move composables inside an activity to their own file using only a "screen" composable in the activity
• the "screen" composable should try to
  • only pass primitive types to its child composable(s)
  • handle view model creation (if possible)

@sunkup this PR is getting quite big, I think we should review it now, and then migrate the remaining activity (should only be EditCalendarActivity I think).

Originally posted by @ArnyminerZ in #318 (comment)