Reconsider the ImportOrder rule of checkstyle

I have been confused many times with the wrong import order checkstyle warnings in the pre commit result. And I haven't found any developer guide which tells me what is the right order so this time I decided to read the rule by myself.

In the ImportOrder section, we declare sortStaticImportsAlphabetically which can only work with option 'top' or 'bottom'(which means place static imports on top or bottom) and use the default option which is 'under'.

I prefer placing static import on top, so I suggest here we set option to 'top'.