PrefService: fix prefs access on Windows

We were mistakenly using Preferences.systemRoot() to access our
preferences paths. This caused terrible test failures on Windows for
Jenkins, due to attempting to modify system registry entires.

The intent of the PrefService was always to manage preferences at a
per-user level, thus all Preferences should come from the userRoot
instead.

Author: hinerm

src/main/java/org/scijava/prefs/DefaultPrefService.java

@@ -226,9 +226,8 @@ public void clear(final Class<?> c) {
 	@Override
 	public void clearAll() {
 		try {
-			final String[] childNames = Preferences.userRoot().childrenNames();
-			for (final String name : childNames)
-				Preferences.userRoot().node(name).removeNode();
+			for (final String name : allPrefs())
+				prefs(name).removeNode();
 		}
 		catch (final BackingStoreException e) {
 			// do nothing
@@ -465,8 +464,12 @@ private Preferences prefs(final Class<?> c) {
 		return Preferences.userNodeForPackage(c == null ? PrefService.class : c);
 	}
 
+	private String[] allPrefs() throws BackingStoreException {
+		return Preferences.userRoot().childrenNames();
+	}
+
 	private Preferences prefs(final String absolutePath) {
-		return Preferences.systemRoot().node(absolutePath);
+		return Preferences.userRoot().node(absolutePath);
 	}
 
 	private String key(final Class<?> c, final String name) {