Move ContentSettingsFragment.isValidPath to helpers and add unit test for it.

app/src/main/java/org/schabi/newpipe/util/FilePathUtils.java

@@ -0,0 +1,22 @@
+package org.schabi.newpipe.util;
+
+import java.io.File;
+
+public final class FilePathUtils {
+    private FilePathUtils() { }
+
+
+    /**
+     * Check that the path is a valid directory path and it exists.
+     *
+     * @param path full path of directory,
+     * @return is path valid or not
+     */
+    public static boolean isValidDirectoryPath(final String path) {
+        if (path == null || path.isEmpty()) {
+            return false;
+        }
+        final File file = new File(path);
+        return file.exists() && file.isDirectory();
+    }
+}