Added People to the Notification API

In order to support people without a URI and further
changes in MessagingStyle, a new person API is
introduced that allows for a richer presentation.
In addition are we now properly supporting people
without a URI, which is useful for non-handheld
clients

Test: runtest -x tests/app/src/android/app/cts/NotificationTest.java
Bug: 63708826
Change-Id: I496c893273803a2ec4fd3a5b731a6b4d483801ea