Finish trampoline activity for ConfirmDeviceCredential immediately

Make sure to finish ConfirmDeviceCredentialActivity directly, and use
Intent.FLAG_ACTIVITY_FORWARD_RESULT, so we can't even end up in a
state where we have the trampoline activity but not the real activity.

Bug: 23849216
Change-Id: I7a5be5af74ca85c11df1f61a69c3fd5cf558e1fb
3 files changed