fix(ngAnimate): do not assume $document[0] exists

[tjokimie]

Use document.body instead of $document[0].body. Enable mocking of $window
without having to add an empty document object into it.

Closes #11964

[Narretz]

@matsko can you take a look at this? I'm not sure how strict we should be with these injections.

[lgalfaso]

I think this is not an issue. If you want to mock $window then you should provide document. Someone else might want to mock $document and if we use document here, then it would be impossible

[tjokimie]

@lgalfaso Previously this has been an issue in $sce and it was fixed in #9663.

[matsko]

The reason why we use $document in the first place is to allow for mocking. This commit further limits things since we have to change the code afterwards to pull document from $window. Unfortunately you'll need to inject both. Sorry :{