chore(Navbar): supplement the default value of --td-navbar-center-width (#3811)

Author: anlyyao

src/navbar/navbar.less

@@ -127,7 +127,7 @@
     position: absolute;
     bottom: 0;
     left: var(--td-navbar-center-left, @navbar-right);
-    width: var(--td-navbar-center-width);
+    width: var(--td-navbar-center-width, 187px); // 默认值取 Iphone6/7/8 屏幕计算的值
     height: @navbar-height;
     line-height: @navbar-height;
     flex: 1;