Only use getSort if it exists

I mistakely claimed that Cirrus's support for getSort didn't need getSort
in core.  It does.  This makes it optional but adds a TODO to remove the
optional-ness when we no longer support versions of core without it.

Bug: 60529
Change-Id: Ibe6fe83bd83ef0b38be46e81d998791d8b4a0684
Merge "Only use getSort if it exists"