FIX: Allow InputSystem class to set runtime runInBackground field

[jfreire-unity]

Description

Some platforms' background behavior isn't reflected in Application.runInBackground. This PR exposes InputSystem.runInBackground to be called by those particular platforms to tell the Input System that the platform runs in the background.

It should be considered for "internal use" only even though it is part of the public API.

Context: This is the alternative for the rejected PR #1648. Specific platforms can call this API as they wish, otherwise, we rely on Application.runInBackground set in the Editor settings for platforms that allow being run in the background.

• Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • FogBugz ticket attached, example ([case %number%](https://issuetracker.unity3d.com/issues/...)).
  • FogBugz is marked as "Resolved" with next release version correctly set.
• 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.

[lyndon-unity]

Seems reasonable but I don't know all the connection between platform packages and the main package to be sure this is the best way to handle it.

[jfreire-unity]

After going through all the options, I decided to go with the Public API direction for now.
There's no "Plugin" concept in the InputSystem where a setting like this should live and it would involve a bigger effort at the moment. This should be the direction to go next.

[BraveSirAndrew]

@jfreire-unity Yo. Sorry to butt in but this is a breaking change for anyone who is implementing the IInputRuntime interface. Just saying. Also hi!