Merge CallControl#reject into CallControl#disconnect

API council reviewed CallControl and suggested that reject should be
merged into disconnect.  After talking it over with the team, we decided
this was a good suggestion.

changes:
- CallControl#Reject API removed
- CallControl#Disconnect API only allows 3 disconnect codes
- Throw an IllegalArgumentException if the client passes an invalid code
- EndCallTransaction adjustments

bug: 265809508
Test: CTS

Change-Id: I2b11ad507a7c280e8447f069c1865f765991468c