cache lastVoipToken and return it when subsequently call to registerVoipToken

[zxcpoiu]

We should prevent register duplicated delegate to PushKit. So if you have registered voipRegistration on the native side, when you call registerVoipToken on the js side again, it will prevent it to register twice, and is a no op.

But for compatibility and convenience, now we cached the latest voip token on the native side each time we received token from PushKit inside didUpdatePushCredentials, and, if user call registerVoipToken and it's registered already, it will fire register event with the latest cached voip token then.

[zxcpoiu]

TL;DR

• We cache the latest voip token on the native side. ( during an app life cycle )
• Now will fire the latest cached token with register when calling registerVoipToken on the JS ( if you have registered )