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

app/src/test/java/org/schabi/newpipe/util/FilePathHelperTest.java

@@ -0,0 +1,32 @@
+package org.schabi.newpipe.util;
+
+import org.junit.Test;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class FilePathHelperTest {
+    @Test
+    public void testIsValidDirectoryPath() throws IOException {
+        // path that exists
+        final File dir1 = Files.createTempDirectory("dir1").toFile();
+        assertTrue(FilePathUtils.isValidDirectoryPath(dir1.getAbsolutePath()));
+
+        // a directory in above path that exists
+        final File subDir = Files.createDirectory(dir1.toPath().resolve("subdir")).toFile();
+        assertTrue(FilePathUtils.isValidDirectoryPath(subDir.getAbsolutePath()));
+
+        // a directory in above path that doesn't exist
+        assertFalse(FilePathUtils.isValidDirectoryPath(dir1.toPath().resolve("not-exists-subdir").
+                toFile().getAbsolutePath()));
+
+        // file is not a valid direcotry path
+        final File tempFile = Files.createFile(dir1.toPath().resolve("simple_file")).toFile();
+        assertFalse(FilePathUtils.isValidDirectoryPath(tempFile.getAbsolutePath()));
+    }
+
+}