ADD: Refactor KeygenViewModel

[johnnyluo]

KeygenFlowViewModel:

1. Create a UseCase for ParticipantDiscovery, remove GlobalScope, remove LiveData, ensure it shuts down when exiting the ViewModel and in other necessary cases
2. Move the generation of serviceName into the UseCase
3. Deal with action field, make it reactive
4. Make isRelayEnabled reactive, along with network switching
5. Add a static MediatorService::start with arguments so that the Intent is formed there
6. Move generateServerPartyId into the UseCase
7. Make state reactive
8. In several places inside state.update, the value for the state is obtained from a variable, although it is available
9. Move the local serverAddress to constants

GeneratingKeyViewModel:

1. Remove suppression.
2. Add @ApplicationContext for the context.
3. Remove the call to .applicationContext.
4. Make TssMessagePuller a UseCase, just like with ParticipantDiscovery; remove the Cache, either entirely or replace it with a Map.