[BasicListItem] Offer vertical fill options for leading/trailing items
These items were previously always resizing the provided components to vertically fill all available space. This was an undocumented behavior, but some apps were relying on it.
I changed this in feb9fef0 to allow custom leading trailing items that do not fill height, but in the process broke the leading/trailing items in the Users KCM and Tok, which were relying on this behavior, and that commit was reverted.
This commit introduces new properties to allow the developer to choose whether leading or trailing items should fill the space vertically, or use a developer-provided height so they can be a fixed size. For backwards compatibility, it defaults to the old fill height behavior.
The test is adjusted to pass now by using this new property.
cc @cblack