Fix SteamVR_Settings being overwritten during asset import

[Peaj]

SteamVR_Settings are currently overwritten on each asset (re)import as mentioned in #949.

Calling Resources.Load during asset import always returns null. SteamVR_Settings.instance is using Resources.Load to load existing settings. SteamVR_CopyExampleInputFiles is calling SteamVR_Settings.instance from DidReloadScripts callback which is first called during asset import and returns null. This leads to the SteamVR_Settings asset getting overwritten every time the project is (re)imported as mentioned in #949.

This PR is delaying the call to SteamVR_Settings.instance in SteamVR_CopyExampleInputFiles that happens directly during DidReloadScripts till the editor update loop where it is safe to execute Resources.Load.

Fixes #949

[keithbradner]

Despite this taking over a year to get in, we appreciate the PR. This fix will be in the next release.