Only set animationClass on dropdown-content when animations are enabled

[nickschot]

This fixes an issue where the dropdown content could be invisible in tests due to the transitioning in class always being set. Could break visual regression testing etc. in cases where the style for this class makes the dropdown invisible

[nickschot]

🤦 oh this was added here as in a fix for app behaviour: #628

Double checking... but the current behaviour is also not great as it can break tests/percy in the way I described

edit: I think this change is still correct for the described use case, keeps the fix intact and makes it render in tests properly again. Separately from this, the original problem that #628 seems to solve still appears to be broken for dropdowns that are rendered in place regardless of this MR.