API tweaks in response to feedback

Update documentation on PhoneAccount#EXTRA_SORT_ORDER to make it more
clear what the sorting order is in edge cases.

Make DisconnectCause#REASON_EMULATING_SINGLE_CALL public, since it might
get propogated up to dialer apps.

Fixes: 169153499
Test: docs only
Change-Id: I7f810f909a8a61b422283c3bafcc97307e4d863b