Add non-standard certicate methods to supplicant.

registerNonStandardCertCallback() allows the framework
to register an INonStandardCertCallback object.

getCertificate() allows callers within
supplicant to retrieve certificates. The current
implementation always retrieves from the framework,
but we will add the ability to get Keystore2
certificates in a later CL.

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 callback.
Change-Id: I440e2d612f36b94efd55bd620cb04387c5493c1d
4 files changed