fix: rename CLOCK_SKEW and separate client/server user case (#863)

* fix: rename CLOCK_SKEW and separate client/server user case

* update clock skew to 20s
diff --git a/google/auth/credentials.py b/google/auth/credentials.py
index 6356f54..8d9974c 100644
--- a/google/auth/credentials.py
+++ b/google/auth/credentials.py
@@ -62,7 +62,7 @@
 
         # Remove 10 seconds from expiry to err on the side of reporting
         # expiration early so that we avoid the 401-refresh-retry loop.
-        skewed_expiry = self.expiry - _helpers.CLOCK_SKEW
+        skewed_expiry = self.expiry - _helpers.REFRESH_THRESHOLD
         return _helpers.utcnow() >= skewed_expiry
 
     @property