FIX: Ensure a specified splitScreenIndex passed in via JoinPlayer is set in PlayerInput

[tspiller]

Description

Manually providing a specified splitScreenIndex via JoinPlayer was not being passed correctly over to the PlayerInput during OnEnable.
This change takes the initially provided splitScreenIndex and sets it manually so it does not default to playerIndex.

Testing status & QA

Wrote one of the TODO tests for PlayerInput which fit the scenario - PlayerInput_CanSetUpSplitScreen_AndManuallyAllocatePlayersToScreens

Overall Product Risks

• Complexity: Low
• Halo Effect: Low

Comments to reviewers

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

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]

Thanks for adding this very complex test :)