chore: Remove wildcard imports

Author: lqiu96

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

@@ -59,7 +59,14 @@
 import java.text.DateFormat;
 import java.text.ParseException;
 import java.text.SimpleDateFormat;
-import java.util.*;
+import java.util.ArrayList;
+import java.util.Calendar;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Date;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
 
 /**
  * ImpersonatedCredentials allowing credentials issued to a user or service account to impersonate