Update _DiffOp to DiffOp following rename in core

Recently the class _DiffOp was renamed to DiffOp (commit
c37b4dc96418faffc7db5e78f79dc762cb83fb0c in core). This patch
updates references to that class.

Bug: 57380
Change-Id: Ic41b9a764391028d2752b033229b1ef878b2cc57