Fix new input system touches on android.

[timbotimbo]

Description

Unity code or UI that uses touch events using the (new) Input System package doesn't work when using this widget.
This PR adds a workaround to make touch input work using the new Input System.

Context

I've been experimenting and this does work in a platform view using Hybrid composition, it is just Virtual display that is broken.
After logging both implementations I noticed that touch events using HC have a positive integer deviceId, which can be anything for different devices.
VD touch events will always have a deviceId of 0.

According to the android docs

An id of zero indicates that the event didn't come from a physical device and maps to the default keymap. The other numbers are arbitrary and you shouldn't depend on the values.

My tests on multiple real devices and emulators showed me values of 2, 3, 4, 12 and 14.

Fix

In onToucEvent change the deviceId of a MotionEvent if it is 0.
I chose -1 as that works in Unity and seems unlikely to conflict with any actual hardware.

The only downside is that we need to copy the full MotionEvent because the deviceId is immutable.
However I believe performance impact is negilible.

Changing the deviceId does not make any difference to the old input system, which likely ignores it anyway.

Testing

• Use the example project.
• Add a canvas with a button. Make sure the eventsystem uses the new input system.
  
  
• Now try pressing the canvas button while running the Flutter app.
• It doesn't work without this fix.

So far I've tested this with Unity 2022.3.21 on:

• Samsung S20FE, Android 13
• Samsung S8, Android 9,
• Wilefox Swift, Android 7,
• Samsung J5 Android 6,

Video

I should probably have used a bigger button, but you can see the fix in these screen recordings.

before.mp4

after.mp4

Just to be sure this PR might need some additional testing on different Android devices and Unity versions.
However I'm confident that deviceId = -1 will be safe for nearly anyone.

[jamesncl]

This looks great, thank you. I'm thinking this could be released as a pre-release version (something like 1.1.0-beta1)

I've tested using 2022.3.20 using the devices I have available, all looks good:

• Pixel 2
• Galaxy S22
• Galaxy Tab A7
• Xiaomi 10T Pro

[jamesncl]

Published as 1.1.0-beta1

dependencies:
  flutter_embed_unity: ^1.1.0-beta1

[timbotimbo]

I just tested and got it working using Unity 2019.3.40, which uses Input system 1.3 from 2022.
Since it works that far back, I don't think there will be compatibility issues based on Unity versions.

[jamesncl]

Great. I'll test it 'in the wild' on one of my apps in a beta track and then hopefully promote it to 1.1.0 production