Replace call into Wifi Keystore with a
call into the new get_certificate method.

Bug: 205764502
Test: Manual test - connect to a WPA Enterprise network.
      Tests that KS2 path is still working.

      Manual test - store a key-value pair to
      legacy Keystore. Check that we can retrieve
      the value in supplicant. Tests the legacy
      keystore path.

      Manual test - try to retrieve a non-existing
      value from legacy keystore. Check that the
      get_certificate method returns -1

Change-Id: I5b595b6592ca9298a2cb667bc38edd4bd2d1e8e6
6 files changed