Fix up EAP-SIM documentation

Address API Council comments on doucmentation for the method calls
and constants related to EAP-SIM.  While here, improve unit tests
to ensure that passing a null certificate (chain) causes the config
to forget any existing client certificates.

Bug: 35847887
Test: Unit tests

Change-Id: I1c4e18e1a7cfb61aa4764e32778793368938e70b