This change makes it so that we track all the motion events encountered by `FlutterView` and all of its subviews in the `MotionEventTracker` class, indexed by a unique `MotionEventId`. This identifier is then passed to the Flutter framework as seen in https://github.com/flutter/flutter/pull/60930. Once the gestures take part in gesture disambiguation and are sent back to the engine, we look-up the original motion event using the `MotionEventId` and dispatch it to the platform.

Bug: https://github.com/flutter/flutter/issues/58837

* apply null safety syntax to mobile dart:ui

* opt out dart:ui from nnbd

* Annotate tests as well

* Moves pointer event sanitizing to engine

* fix comment format

* fix formatting

* addressing comment

* fix format

* fix format

* addressing comment

Dart lints added:
* Avoid optional new
* Avoid optional const
* Prefer single quotes
* Prefer default assignment `=`

Adds support for pointer signals, in a way that will support both discrete events (e.g., scroll wheels, flutter/flutter#22762) and continuous gestures (e.g., trackpad scroll, flutter/flutter#21953).

Also exposes these new event options to the embedder. Does not include code to send the
new events from the platform shells.

Instead, send them with the new unknown PointerDeviceKind.

We hit this when running `adb shell input tap` in tests which sends events with
an unknown tool type.

This also fills in a missing conversion for TOOL_TYPE_ERASER.

This name is consistent with how the other `dart:` libraries are named now.

These are implemented on macOS and Fuchsia.

To make the default state all zeros.

Fixes https://github.com/flutter/flutter/issues/6236

Rather than using mojom to encode pointer data, we now encode and decode it
manually. A future patch will remove the mojom codepath once the framework is
updated.