Improve DisconnectCause API for mainline.

As part of API feedback, improve the DisconnectCause API that was
formalized as part of Telecom mainline. This CL has some minor cleanup
around those suggestions (refer to the associated bug for more details).

Bug: 327030432
Bug: 311773409
Test: atest TelecomUnitTests, atest DataObjectUnitTests
Change-Id: Ic1dd1ae2221be21207474950f307ba72fe8b35af