rely on gax for validation

Author: mpeddada1

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

@@ -141,11 +141,7 @@ private ServiceAccountJwtAccessCredentials(
     this.defaultAudience = defaultAudience;
     this.credentialsCache = createCache();
     this.quotaProjectId = quotaProjectId;
-    if (universeDomain == null || universeDomain.trim().isEmpty()) {
-      this.universeDomain = Credentials.GOOGLE_DEFAULT_UNIVERSE;
-    } else {
-      this.universeDomain = universeDomain;
-    }
+    this.universeDomain = universeDomain;
   }
 
   /**
@@ -437,6 +433,7 @@ public final String getPrivateKeyId() {
     return privateKeyId;
   }
 
+  /** Returns the universe domain (example, googleapis.com) for the credentials instance. */
   @Override
   public final String getUniverseDomain() {
     return universeDomain;
@@ -568,6 +565,7 @@ public Builder setQuotaProjectId(String quotaProjectId) {
     }
 
     @CanIgnoreReturnValue
+    /** Sets the universe domain (example, googleapis.com). */
     public Builder setUniverseDomain(String universeDomain) {
       this.universeDomain = universeDomain;
       return this;
@@ -597,6 +595,7 @@ public String getQuotaProjectId() {
       return quotaProjectId;
     }
 
+    /** Returns the universe domain (example, googleapis.com) for the credentials instance. */
     public String getUniverseDomain() {
       return universeDomain;
     }

oauth2_http/javatests/com/google/auth/oauth2/ServiceAccountJwtAccessCredentialsTest.java

@@ -113,7 +113,7 @@ public void constructor_allParameters_constructs() throws IOException {
     assertEquals(privateKey, credentials.getPrivateKey());
     assertEquals(SA_PRIVATE_KEY_ID, credentials.getPrivateKeyId());
     assertEquals(QUOTA_PROJECT, credentials.getQuotaProjectId());
-    assertEquals(GOOGLE_DEFAULT_UNIVERSE, credentials.getUniverseDomain());
+    assertNull(credentials.getUniverseDomain());
   }
 
   @Test
@@ -917,7 +917,7 @@ public void builder_defaultUniverseDomain() throws IOException {
             .setPrivateKeyId(SA_PRIVATE_KEY_ID)
             .setDefaultAudience(URI.create("default-audience"))
             .build();
-    assertEquals(GOOGLE_DEFAULT_UNIVERSE, credentials.getUniverseDomain());
+    assertNull(credentials.getUniverseDomain());
   }
 
   @Test