Clarifies requirements of registerIntent API

- Clarifies that only one PendingIntent can be registered to a client
  at a time, since otherwise it can't be regenerated meaningfully.
- Also adds that a ContextHubInfo object is provided in Intent events
  to regenerate the client.

Bug: 117612105
Test: None
Change-Id: Ia2b1cc9bc01d10b092598374c0d82e4fa1b45079