This method will let us remove the activity service.

The platform use cases are handled by the PlatformPlugin. The media service and
the sensor service have no clients. If we want to use them, we should create
plugins for them.

Fixes https://github.com/flutter/flutter/issues/6303

Fixes https://github.com/flutter/flutter/issues/6152

Also, expose a convenience class for processing JSON messages.

The Skia team tells us that these go faster.

After this patch, the PlatformPlugin has all the required functionality on Android.

The PlatformPlugin is the first of the built-in plugins that will replace the
mojom platform services.

See https://github.com/flutter/flutter/issues/6274

After this patch, platform messages now take the same path through the system
that semantics data does (on Android). Support on iOS will be in another patch.

We've cut over to the new semantics interface.

TalkBack still isn't fully working, but this patch gets us much closer.

Previously the Dart entry points weren't wired up.

This fixes this Fuchsia build, which uses the latest //lib/ftl.

This transport uses the Dart and JNI APIs directly instead of indirecting
through Mojo.

We only ever had one UIDelegate. We might as well have people use the Engine
interface directly.

This allows setting testonly=true for gtest in the Fuchsia build.

This patch moves the rest of the clients to use the new PointerData
transport.

This patch prepares us to switch sending semantics information over mojom to
using dart:ui directly. Currently the recorded information is dropped on the
floor, but that will improve in future patches.

Example: $ mojo:launcher mojo:spinning_square

Instead, clients should use host messaging.

We now use JNI rather than Mojo to transport pointer data from Java to Dart.
Also, fill in a few more of the pointer data fields from information in Java.