Introduce simplified Default credential provision

As the first step in credential consolidation, we replace
injection of application default credential in for KMS and
Drive.

Tests:
- for Drive, tested with exportDomainLists and exportReservedTerms.
- For KMS, used CLI commands (get_keyring_secret and update_kms_keyring) to change and
  restore secret for one key.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=211819859

javatests/google/registry/export/ExportDomainListsActionTest.java

@@ -64,7 +64,7 @@ public class ExportDomainListsActionTest extends MapreduceTestCase<ExportDomainL
     persistResource(Registry.get("tld").asBuilder().setDriveFolderId("brouhaha").build());
     persistResource(Registry.get("testtld").asBuilder().setTldType(TldType.TEST).build());
 
-    ExportDomainListsReducer.setDriveConnectionForTesting(driveConnection);
+    ExportDomainListsReducer.setDriveConnectionForTesting(() -> driveConnection);
 
     action = new ExportDomainListsAction();
     action.mrRunner = makeDefaultRunner();