Play Framework

[PF-30] Delete FlashPolicyHandler; introduce PlayChannel abstraction

Phase 1 + Phase 2 of the Netty 3.10.6 -> Netty 4.2.x migration plan, both
of which are non-destructive prep work that ships independently of the
codec swap.

Phase 1 - delete FlashPolicyHandler.
Adobe Flash has been EOL since December 2020. The handler responded to
crossdomain.xml policy requests on the HTTP port and is dead code in
modern deployments. Removed the source file and its entry in the default
play.netty.pipeline / play.ssl.netty.pipeline strings (in both factory
classes and the application-skel comments).

Phase 2 - introduce PlayChannel abstraction.
Netty 3's ChannelHandlerContext currently leaks into the public API via
Http.Inbound and F.BlockingEventStream constructors. To unblock a future
Netty 4 cutover without breaking applications that subclass Http.Inbound
or use BlockingEventStream directly, add a single-method PlayChannel
interface (setReadable(boolean) - the only operation the WebSocket
backpressure code needs) plus a transitional NettyPlayChannel impl.

The Netty-typed constructors are kept and marked @Deprecated; they
forward to the new PlayChannel-typed constructors via NettyPlayChannel.
Internal callers (PlayHandler) now use the non-deprecated path.

Verified with `ant clean unittest` (295 tests, 0 failures).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>