Fixes https://github.com/flutter/flutter/issues/10569

If it is not invoked, it triggers a fatal fidl runtime check.

Rather than receiving an ApplicationEnvironment, we now receive a namespace.
Once we have FIDL2, we can plumb the namespace all the way through to Dart.

We still don't get IME text, but at least we don't crash.

This patch wires Mozart's text input system to flutter/textinput. This patch
doesn't fully work yet because the Mozart side isn't fully implemented.

Instead of updating to match new HID touchscreen hacks exposed by
magenta, just remove the direct input layer entirely.  Mozart all the
time from now on.

This version does not recycle buffers. It is meant only as an initial solution,
and the proper buffer recycling logic has been broken out into a separate change

Updating SetListener to SetEventListener which comes with a new SetHitViewTester

Mozart ADD, REMOVE and HOVER phases were going through the default value
of Blink's Cancel type, causing inconsistencies.

Bug: US-157

* New mozart input events.

* Adding TODO for keyrepeat

[content_handler] Make RuntimeHolder convey its viewport metrics to the direct input instance. (#3367)

This interface has been replaced with HandleWaiter, which is the new hotness.

These are implemented on macOS and Fuchsia.

Fixes https://github.com/flutter/flutter/issues/6631

Previously we treated everything as touch.

We no longer use mojom to transport messages. We still use the Mojo EDK
to spin the event loop, however.

Now we just pass the data directly.

Previously, these signals were sent over Mojo. Now we send them through
platform messages.

We no longer offer mojo handles via this mechanism.