[Splitter] Pane size not persisted on collapse/expand

[mbechev]

Describe the bug
Splitter panes size are not persisted when the collapsed state is toggled and in case the first pane has the initial size set.
After expanding the panes, the initially set size of the first pane is reset and changed.

Collapse the right pane to reproduce the issue:
https://stackblitz.com/edit/angular-x2krsr-w6rr5d?file=src%2Fapp%2Fapp.component.ts

If the split bar is resized a bit, the panes size is respected and the issue is not reproduced.