[CrOS Tether]: Add LocalDeviceDataProvider, which fetches device data for the local device.

components/cryptauth/cryptauth_enrollment_manager.cc

Index: components/cryptauth/cryptauth_enrollment_manager.cc
diff --git a/components/cryptauth/cryptauth_enrollment_manager.cc b/components/cryptauth/cryptauth_enrollment_manager.cc
index 6da6468eea32fa70f3e32f53bb1147589fb6f60c..5a3befac04537e7352c8ebbcc2a937f15254861b 100644
--- a/components/cryptauth/cryptauth_enrollment_manager.cc
+++ b/components/cryptauth/cryptauth_enrollment_manager.cc
@@ -167,7 +167,7 @@ CryptAuthEnrollmentManager::CreateSyncScheduler() {
       kEnrollmentMaxJitterRatio, "CryptAuth Enrollment");
 }
 
-std::string CryptAuthEnrollmentManager::GetUserPublicKey() {
+std::string CryptAuthEnrollmentManager::GetUserPublicKey() const {
   std::string public_key;
   if (!base::Base64UrlDecode(
           pref_service_->GetString(prefs::kCryptAuthEnrollmentUserPublicKey),
@@ -178,7 +178,7 @@ std::string CryptAuthEnrollmentManager::GetUserPublicKey() {
   return public_key;
 }
 
-std::string CryptAuthEnrollmentManager::GetUserPrivateKey() {
+std::string CryptAuthEnrollmentManager::GetUserPrivateKey() const {
   std::string private_key;
   if (!base::Base64UrlDecode(
           pref_service_->GetString(prefs::kCryptAuthEnrollmentUserPrivateKey),