EMSUSD-1718 automatically close prefs when changing units

When the USD import needs to change the Maya units preferences, we need
to close the preferences window, otherwise it will overwrite the preferences
when closed.

lib/mayaUsd/fileio/jobs/readJob.cpp

@@ -756,7 +756,15 @@ void UsdMaya_ReadJob::_ConvertUpAxisAndUnitsByChangingMayaPrefs(
         } else {
             const MString mayaUnitText = UsdMayaUtil::ConvertMDistanceUnitToText(mayaUnit);
             MString       changeUnitsCmd;
-            changeUnitsCmd.format("currentUnit -linear ^1s;", mayaUnitText);
+            changeUnitsCmd.format(
+                "global string $gPreferenceWindow;\n"
+                "if (`window -query -exists $gPreferenceWindow`) {\n"
+                "    if (`window -query -visible $gPreferenceWindow`) {\n"
+                "        window -edit -visible off $gPreferenceWindow;\n"
+                "    }\n"
+                "}\n"
+                "currentUnit -linear ^1s;",
+                mayaUnitText);
 
             // Note: we *must* execute the units change on-idle because the import process
             //       saves and restores all units! If we change it now, the change would be lost.