WebSocketModule uses OkHttpClientProvider

Summary:
Instead of creating a brand-new OkHttpClient.Builder() per WebSocket
connection, derive from OkHttpClientProvider.getOkHttpClient() so
that all connections share the same connection pool and dispatcher.

Also remove the dispatcher shutdown call, since the dispatcher is now
shared with the global OkHttpClient and shutting it down would kill
all connections.

Changelog: [Internal]

Differential Revision: D95059116