Make SidebarExpandWidget work in RTL

Rename collapse/expand.svg to collapse-ltr/collapse-rtl.svg so
CSSJanus's filename munging will flip them in RTL.

Make the icon appear on the right in RTL by setting text-align: left;.
This doesn't change anything in LTR, but becomes text-align: right;
in RTL.

Bonus: @embed the collapse/expand icons

Bug: T110488
Change-Id: I7a9643c0a3a018681ce02075d58ace6d21f7bd59