FIX: Fixed errors caused by recreation of DefaultInputActions instance in InputSystemProvider

[Secticide]

Description

Fixes ISXB-1446

Errors similar to the following would be output when running playmode tests when using the InputTestFixture (with domain reloads on playmode entry turned off):

NullReferenceException: Object reference not set to an instance of an object
UnityEngine.InputSystem.InputActionState+BindingState.get_actionIndex () (at ./Library/PackageCache/com.unity.inputsystem@e2c83221d2dc/InputSystem/Actions/InputActionState.cs:3443)
UnityEngine.InputSystem.InputActionState.ProcessControlStateChange (System.Int32 mapIndex, System.Int32 controlIndex, System.Int32 bindingIndex, System.Double time, UnityEngine.InputSystem.LowLevel.InputEventPtr eventPtr) (at ./Library/PackageCache/com.unity.inputsystem@e2c83221d2dc/InputSystem/Actions/InputActionState.cs:1442)
UnityEngine.InputSystem.InputActionState.UnityEngine.InputSystem.LowLevel.IInputStateChangeMonitor.NotifyControlStateChanged (UnityEngine.InputSystem.InputControl control, System.Double time, UnityEngine.InputSystem.LowLevel.InputEventPtr eventPtr, System.Int64 mapControlAndBindingIndex) (at ./Library/PackageCache/com.unity.inputsystem@e2c83221d2dc/InputSystem/Actions/InputActionState.cs:1343)
UnityEngine.InputSystem.InputManager.FireStateChangeNotifications (System.Int32 deviceIndex, System.Double internalTime, UnityEngine.InputSystem.LowLevel.InputEvent* eventPtr) (at ./Library/PackageCache/com.unity.inputsystem@e2c83221d2dc/InputSystem/InputManagerStateMonitors.cs:394)
UnityEngine.InputSystem.LowLevel.<>c__DisplayClass7_0:<set_onUpdate>b__0(NativeInputUpdateType, NativeInputEventBuffer*)
UnityEngineInternal.Input.NativeInputSystem:NotifyUpdate(NativeInputUpdateType, IntPtr)

This was caused by the recreation of a DefaultInputActions instance via InputSystemProvider.OnActionsChange.

This PR makes some adjustments to InputSystemProvider to only create a single DefaultInputActions instance that is reused. However, when listening for changes to actions - we still reselect the InputActionAsset used on every change.

I also noticed a bunch of things that could be cleaned up in the process and so made some modifications and renames to the class:

• InputSystemProvider.Configuration was only holding the InputActionAsset, the action strings were constant. Since the InputActionAsset reference was removed - it made sense to switch it to being a static class.
• There was a name mismatch between RegisterNextPreviousAction & UnregisterFixedActions and so RegisterNextPreviousAction has been renamed to RegisterFixedActions.
• RegisterFixedActions & UnregisterFixedActions were being called on every action update, this is much more than required so they have been moved to Initialize and Shutdown respectively.

Testing status & QA

Tested locally using the project provided by the user.

Overall Product Risks

• Complexity: 1
• Halo Effect: 1

Checklist

Before review:

• Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • JIRA ticket linked, example (case %%). If it is a private issue, just add the case ID without a link.
  • Jira port for the next release set as "Resolved".
• Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.

After merge:

• Create forward/backward port if needed. If you are blocked from creating a forward port now please add a task to ISX-1444.

[ekcoh]

Added @Pauliusd01 for QA perspective.

[Pauliusd01]

LGTM, checked the user project, made sure that UITK UI still works as expected in playmode and win player, project wide actions still work as expected, mentioned bugs in the comments (ISX-1954, ISXB-811) are still fixed