feat: Modify the refresh window to match go/async-token-refresh. Serv…

…erless tokens are cached until 4 minutes before expiration, so 4 minutes is the ideal refresh window. (#1352)

* feat: Modify the refresh window to match go/async-token-refresh. Serverless tokens are cached until 4 minutes before expiration, so 4 minutes is the ideal refresh window.
* Tweak time to 3 minutes and 45 seconds, giving the MDS a 15 second window to refresh.
* Update both expiration margin and refresh margin.

oauth2_http/java/com/google/auth/oauth2/OAuth2Credentials.java

@@ -68,8 +68,8 @@
 public class OAuth2Credentials extends Credentials {
 
   private static final long serialVersionUID = 4556936364828217687L;
-  static final Duration DEFAULT_EXPIRATION_MARGIN = Duration.ofMinutes(5);
-  static final Duration DEFAULT_REFRESH_MARGIN = Duration.ofMinutes(6);
+  static final Duration DEFAULT_EXPIRATION_MARGIN = Duration.ofMinutes(3);
+  static final Duration DEFAULT_REFRESH_MARGIN = Duration.ofMinutes(3).plusSeconds(45);
   private static final ImmutableMap<String, List<String>> EMPTY_EXTRA_HEADERS = ImmutableMap.of();
 
   @VisibleForTesting private final Duration expirationMargin;