Add ListEnabledProviders API method for Credential Manager

Adds ListEnabledProviders API which is a SystemApi used
by Settings and Chrome to list the enabled and user
visible installed providers.

Test: make
Bug: 253157179
CTS-Coverage-Bug: 247549381
Change-Id: I25ef3a294f051855c0a4ae104d1111f1e8b8529d