MaterialCollapsible setting active index with 1 as start

[wassx]

It seems pretty uncommon why there is a range start from 1 for the index.
GWT.log("The active index must be a one-base index to mark as active.", new IndexOutOfBoundsException());
What is the reason for this design please?

[kevzlou7979]

Patched the issue and standardized the collapsible item to have a 0-starting based index. 60d3d21

Please review the changes and might be good to test it out too.

[ingosch]

setActive(0) is not working cause of condition "index > 0" in line 258. Must be "index >= 0".