Add logic to retrieve KS2 certificates in supplicant.

Logic was largely copied from
android.system.wifi.keystore@1.0

Bug: 205764502
Test: Manual test - store a key-value pair to
      legacy Keystore. Check that we can retrieve
      the value in supplicant using the new method.
Change-Id: I8f12e447747f2e1a98e2d37b3453526cdde9e2de
7 files changed