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
6 files changed