seperating callback changes

After some discussion amongst Telecom and an API council member, the
better design decision is to break up previous CallEventCallback into
two seperate callbacks.

The reason being, there is one set of callbacks that require the
client to complete a Consumer and another set of callbacks that only
provide event updates.

By making this change, the API surface becomes cleaner and should
eliminate some confusion.

bug: 268098087
Test: CTS
Change-Id: Ib9dce46096bd26b33221501c3ac8240670ca27ab