AvoidXferMode -> AvoidXfermode.

When I moved this code here from external/skia, I accidentally changed
the lowercase 'm' to uppercase 'M'. Change back, to keep consistent
with SkXfermode and Xfermode.cpp.

Change-Id: I9694d29a22aaed7fcc8708f5962e3d2032297d08
4 files changed