Flutter migration

[Izchomatik]

This is the main issue for roc-droid Flutter migration.

Sub-issues:

• [x] #95
• [x] #96
• [x] #97
• [x] #100
• [x] #103
• [x] #104
• [x] #99
• [ ] #98
• [ ] #109
• [ ] #106
• [ ] #105

[Izchomatik]

@gavv and @ortex - this is Flutter migration plan for your review

[ortex]

The plan looks good to me 😃 here is the branch for development https://github.com/roc-streaming/roc-droid/tree/flutter