[AWARE] Update API per review

- Move Handler to last position
- Peer ID changed from 'Object' -> opaque class
- Rename onMessageSent -> onMessageSendSucceeded

(cherry pick of commit 1fef08507aeb49bb2e4e7be4804a378ba2cf7770)

Bug: 31470256
Test: unit tests & integration (sl4a) tests pass
Change-Id: I5530d310e982ea16a63a5af1f704625fd24e436c