Credential management app follow ups

* KeyChain API to check if the caller is the
  credential management app.
* KeyChain API to get the authentication policy
  if the caller is the credential management app.
* KeyChain createManageCredentialsIntent docs
  mention startActivityForResult should be used

Bug: 177979648
Test: atest android.devicepolicy.cts.CredentialManagementAppTest
Change-Id: Ia5125adb677ec103a9d5a5318edf95050e74916e