[Velocity Tracker] Fix: Issue 97761: Flutter Scrolling does not match iOS; inadvertent scrolling when user lifts up finger (#132291)

## Issue

**Issue:** https://github.com/flutter/flutter/issues/97761

https://github.com/flutter/flutter/assets/1863934/53c5e0df-b85a-483c-a17d-bddd18db3aa9

## The Cause:

The bug is very simple to understand - `velocity_tracker.dart` **only adds new samples while your finger is moving**.

**Therefore**, if you move your finger quickly & (important) stop suddenly with no extra movement, the last 3 samples will all be > 0 dy. Regardless of how long you wait, you will get movement when you lift up your finger.

**Logs from velocity_tracker.dart:**
Notice: all 3 `_previousVelocityAt` are `dy > 0` despite a 2 second delay since the last scroll
```
// start moving finger
flutter: addPosition dy:-464.0
flutter: addPosition dy:-465.0
flutter: addPosition dy:-466.0
flutter: addPosition dy:-467.0
flutter: addPosition dy:-468.0
flutter: addPosition dy:-469.0
flutter: addPosition dy:-470.0
// stop moving finger here, keep it still for 2 seconds & lift it up
flutter: _previousVelocityAt(-2) samples(-467.0, -468.0)) dy:-176.772140710624
flutter: _previousVelocityAt(-1) samples(-468.0, -469.0)) dy:-375.0937734433609
flutter: _previousVelocityAt(0) samples(-469.0, -470.0)) dy:-175.71604287471447
flutter: primaryVelocity DragEndDetails(Velocity(0.0, -305.5)).primaryVelocity
flutter: createBallisticSimulation pixels 464.16666666666663 velocity 305.4699824197211
```

## The Fix

**There are 3 options to fix it:**
A. sample uniformly *per unit time* (a larger more risky change, hurts battery life)
B. consider elapsed time since the last sample. If greater than X, assume no more velocity. (easy & just as valid)
C. similar to B, but instead add "ghost samples" of velocity zero, and run calculations as normal (a bit tricker, of dubious benefit imo)

**For Option B I considered two approaches:**
1. _get the current timestamp and compare to event timestamp._  This is tricky because events are documented to use an arbitrary timescale & I wasn't able to find the code that generates the timestamps. This approach could be considered more.
2. _get a new timestamp using Stopwatch and compare now vs when the last sample was added._ This is the solution implemented here.  There is a limitation in that we don't know when addSamples is called relative to the event. But, this estimation is already on a very low latency path & still it gives us a *minimum* time bound which is sufficient for comparison. 

**This PR chooses the simplest of the all solutions. Please try it our yourself, it completely solves the problem 😀** Option _B.1_ would be a nice alternative as well, if we can define and access the same timesource as the pointer tracker in a maintainable simple way.

## After Fix

https://github.com/flutter/flutter/assets/1863934/be50d8e7-d5da-495a-a4af-c71bc541cbe3