redux types: Make PresenceState inexact, for now.

Until Flow v0.111.0 or later, where we'll be when we upgrade to
React Native v0.62.0 (that's issue zulip#3782), exact object types with
indexer properties are broken. See more details in
src/utils/jsonable.js.

In particular, this will help us get userHelpers-test.js
type-checked.

src/reduxTypes.js

@@ -211,9 +211,9 @@ export type OutboxState = Outbox[];
  * @prop (email) - Indexes over all users for which the app has received a
  *   presence status.
  */
-export type PresenceState = {|
+export type PresenceState = {
   [email: string]: UserPresence,
-|};
+};
 
 /**
  * State with miscellaneous data from the server; our state subtree `realm`.