Documentation and symbolic constants for dispatcher actions (#31278)

* Remove unreachable code

`view_last_screen` is never used.

* Remove unused action `user_activity_started`

Nothing listens to this, so it's pointless.

* Symbolic constant for `Action.UserActivity`

* Define symbolic constants for more `Action`s

Constants for some actions that are emitted by `Lifecycle`

src/BasePlatform.ts

@@ -88,7 +88,7 @@ export default abstract class BasePlatform {
 
     protected onAction(payload: ActionPayload): void {
         switch (payload.action) {
-            case "on_client_not_viable":
+            case Action.ClientNotViable:
             case Action.OnLoggedOut:
                 this.setNotificationCount(0);
                 break;