What do I have to add to a layout to hide the titlebar?

If you mean the title in the grey bar just below the notification bar, you have to fill the "label" property of the activities you want not to display the application title in your manifest. You can then either define a title for the current activity of simply give an empty string. does it help?

<activity android:name="FirstActivity" android:label="it's title different from application's one"></activity>

Whenever I do the suggested changes, and then run rake run:android, each time my AndroidManifest file gets modified to its previous state.

I got the solution as,

Update your build.yml file as

android
  android_title: 0

This worked for me.