Currently, Titanium's ButtonBar is an iOS only feature. We should add support on Android for parity.
On iOS, this feature is implemented via the UISegmentedControl whose "momentary" style is set to true. This means the UISegmentedControl doesn't maintain a check/selection state and acts like buttons.
On Android and Windows, this should be implemented as a horizontal layout of buttons.
The ButtonBar "style" property has been deprecated on iOS since Titanium 3.4.2 and will log a warning when you attempt to set it. iOS currently ignores this property. We can re-purpose this property to implement the 3 different button styles documented by Google's material theme: "Text Button", "Outlined Button", and "Contained Button".