Show Toast when download starts

Add toast to inform the user that download started and add the right string in values.

app/src/main/java/org/schabi/newpipe/download/DownloadDialog.java

@@ -672,6 +672,9 @@ public class DownloadDialog extends DialogFragment
         prefs.edit()
                 .putString(getString(R.string.last_used_download_type), selectedMediaType)
                 .apply();
+
+        Toast.makeText(context, getString(R.string.download_has_started),
+                Toast.LENGTH_SHORT).show();
     }
 
     private void checkSelectedDownload(final StoredDirectoryHelper mainStorage,

app/src/main/res/values/strings.xml

@@ -713,4 +713,5 @@
     <string name="auto_device_theme_title">Automatic (device theme)</string>
     <string name="night_theme_summary">Select your favorite night theme — %s</string>
     <string name="select_night_theme_toast">You can select your favorite night theme below</string>
+    <string name="download_has_started">Download has started</string>
 </resources>