Remove hardcoded URL string

* Details on URLs in the credential management
  app preference should be translatable.
* Remove hardcoded text and introduce strings.

Manual testing
* Download TestDPC and select 'Request to
  manage credentials'.
* Select allow.
* Go to Settings Credential management app
  preference and verify URL strings are displayed
  correctly.

Bug: 170627997
Test: manual testing
Change-Id: Iadff0c136064dabc3f77e30b46356126d657397f