Add ITelephony setCepEnabled command line command.

This new command line command is used to disable conference event package
handling so that we can test behavior where CEP is not present on a
network, even though a network actually supports it.

Test: Disabled CEP using command line tool, verified bug disposition.
Bug: 139299700
Change-Id: I9f2258ddfbe982159de0f37d4a865e2511079c72