Revert of remove unused SkAvoidXfermode

Revert of remove unused SkAvoidXfermode (patchset #2 id:20001 of https://codereview.chromium.org/860583002/)

Reason for revert:
Breaking Android framework build, which still uses AvoidXfermode. (failure shown here: https://android-build.storage.googleapis.com/builds/git_master-skia-linux-razor-userdebug/1689202/565b76b4bd8cc2ffd5aeb37e524f6a35e9b8791995d6e8c719a3a6d83aece936/logs/build_error.log?Signature=lHYyiconR%2BoTzY9ILAKlaUm6nFnpJQPTI4XUei1ZccBRyz6PxspCNvYCDO5za92yZSoy9refHT7uh2Hxlxew1ZBePxKCp2nnwJB7a%2B403kiHMHHawjecsbPnTE9QNNO8XMoqAjujcAplorhwsIBZYOCxb%2Bo5010BCfzTd%2F8GFa8%3D&Expires=1421764644&GoogleAccessId=701025073339-mqn0q2nvir9iurm6q5d00tdv7blbgvjr%40developer.gserviceaccount.com)

Here is the Java class that uses it: https://cs.corp.google.com/#android/frameworks/base/graphics/java/android/graphics/AvoidXfermode.java&sq=package:android

It has been deprecated. The glue code (which actually references the Skia file) is here: https://cs.corp.google.com/#android/frameworks/base/core/jni/android/graphics/Xfermode.cpp&q=Xfermode.cpp&sq=package:android

Mike suggested we move the implementation into Android, which is fine with me.

Original issue's description:
> remove unused SkAvoidXfermode
> 
> BUG=skia:
> TBR=
> NOTRY=True
> 
> Committed: https://skia.googlesource.com/skia/+/2d02df7a9a50217dac8c1d11681832a927ed79ce

TBR=bsalomon@google.com,reed@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Committed: https://skia.googlesource.com/skia/+/b2b416d3847c138a0a13876fdc1cdeae607b7e31

[scroggo, 2015-01-20 14:32:47]

Created Revert of remove unused SkAvoidXfermode

[commit-bot: I haz the power, 2015-01-20 14:33:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/822953003/1

[commit-bot: I haz the power, 2015-01-20 14:33:17]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/b2b416d3847c138a0a13876fdc1cdeae607b7e31

[scroggo, 2015-01-30 13:58:54]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/890893003/ by scroggo@google.com.

The reason for reverting is: Relanding, now that SkAvoidXfermode has been moved
into Android..