Move SkAvoidXferMode into frameworks/base.

We are removing it from Skia, so we need it here to support Android.

Add some small cleanups (remove comment that doesn't apply, convert
SK_OVERRIDE to override, remove 'virtual' keyword from methods with
'override' on them).

BUG:skbug.com/3329
Change-Id: I1f883082d6fb9d49b9c9ba9e1f50bd713eabf915