[TOFU] Harden certificate subject parsing

Harden certificate subject parsing:
* Handle escaped commas correctly.
* Ignore duplicates and unknowns

Bug: 274743014
Test: atest CertificateSubjectInfoTest WifiConfigManagerTest
Test: Use TOFU with certificates that have a comma in the
common name and other fields, verify UI displays correct and full
information.

Change-Id: I5d716637104eac9e7104bcc80b9c327f4b7f1b16
4 files changed