Split AlphaThresholdEffect out into its own file

Mainly mechanical although I did:
   convert Create method to sk_sp Make version
   rm extra '\t's and '\n's in emitCode

TBR=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1893193002

Review URL: https://codereview.chromium.org/1893193002
diff --git a/tests/FlattenableFactoryToName.cpp b/tests/FlattenableFactoryToName.cpp
index a22f0b1..35ec984 100644
--- a/tests/FlattenableFactoryToName.cpp
+++ b/tests/FlattenableFactoryToName.cpp
@@ -7,6 +7,7 @@
 
 #include "SkAlphaThresholdFilter.h"
 #include "SkImage.h"
+#include "SkRegion.h"
 #include "Test.h"
 
 static void test_flattenable(skiatest::Reporter* r,