libbrillo: policy: Add GetSecondFactorAuthenticationMode for new device policy

Add a new helper function and the matching unit-tests for the the new
device_second_factor_authentication device policy/proto used to control
how the machine secure element hardware can be used to provide a
second-factor authentication in addition to the TPM functionality.

BUG=b:35545754
TEST='FEATURES=test emerge-eve protofiles libbrillo'
CQ-DEPEND=CL:548356

Change-Id: I9568067a7530c9c0c84142714eb7a67d13e3e45b
Reviewed-on: https://chromium-review.googlesource.com/548559
Commit-Ready: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
6 files changed