Use correct state when calling getZ

When calling traverseInZOrder, there's a call to getZ. Currently
the code uses the drawingState but that's incorrect since sometimes we
traverse the currentState. Changed the getZ function so it uses the
correct state.

Bug: 143200062
Test: Builds and runs
Change-Id: Ifcd9bafbf0906b4f6f8f68f7d20d1a1025cef449