applets/systemtray: Fix wrong layout when loading

Do not enable the add and display animations on the grid layout until the component is ready.

This fixes some of the items getting stuck on the wrong position when loading the systemtray (on plasmashell start), probably because some animations were getting cancelled by other ones.

BEFORE	AFTER

I haven't seen any reports about this issue, but I've been experiencing it consistently on every plasmashell start/restart.