Create ContentTransformer to access ContentHandler::preSaveTransform through the service.
Prepare object to hold a data that required for ContentHandler::preSaveTranform params.
This will require making a semi-backwards-incompatible
change no matter what, we don't really have a great way
of hard-deprecating overriding methods.
However, with the ContentHandler calling Content and
Content calling ContentHandler, and with the ProxyContent
trick to stop infinite recursion, it doesn't matter whether
callers use Content or ContentHandler. This will allow us
to naturally convert all callers. But won't really allow
hard-deprecation.
Bug: T287156
Change-Id: If6a2025868ceca3a3b6f11baec39695e47292e40
b782a7e66d