FIX: Set active input handler to InputManager when package is removed (ISXB-675)

[ritamerkl]

Description

This change aims to fix this bug ticket, where the active input handling options can be set independently of the presence of the package.
Now the input handling is set to InputManager once the package is removed. The second part of this fix lands on trunk, where setting the input handling to "InputSystem" or "Both" can only be done when the package is installed.

The fix is available vor Unity version 2020.2 and above, for the sake of the PackageManager package register event being only available from 2020.2 and above.

Testing status & QA

Manual testing.

Overall Product Risks

• Complexity: low
• Halo Effect: low risk

Comments to reviewers

Please describe any additional information such as what to focus on, or historical info for the reviewers.

Checklist

Before review:

• [ x] 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.

[jfreire-unity]

Nice one, thanks for dealing with this. Code looks fine to me.
I only made some nitpick suggestions in some comments text and the dialog text. Happy to approve if QA goes well.

[Pauliusd01]

After latest commit getting this

[Pauliusd01]

Approving only if the last commit is fixed. Here's what I checked:

Removing the IS package with each of the 3 backend settings.
Updating the package instead of removing it (it did not mess with the backend as expected)
Removing the package through the manifest file while the Editor is open (the backend was changed correctly)
Removing the package through the manifest file while the Editor is closed (the backend did not change but that seems to be known)