update CallScreeningService#setSkipCallLog to address 3rd pty apps

A 3rd party developer reported
CallScreeningService.CallResponse#setSkipCallLog(true) was still
logging the blocked call to Dialer.  This turned out to be true.

Turns out we do not allow 3rd party devs from toggling the
setSkipCallLog function.  Only carriers are allowed to toggle
the function.

To make this clear, the docs are being updated.

bug: 238892157
     130213778 (original)

Test: not required; docs only change.
Change-Id: I6b796421f7f178808eab301e12a4367b67ceaa71