[TIMOB-26289] Android: Expose "listSeparatorInsets" for parity

On iOS, there is the [listSeparatorInsets](https://docs.appcelerator.com/platform/latest/#!/api/Titanium.UI.ListView-property-listSeparatorInsets) property of the Ti.UI.ListView to define the padding of the separator insets of each cell. On Android, the same [is possible](https://stackoverflow.com/a/16768738/5537752) but currently not exposed. For parity sake, this should be exposed. I tried to expose the API [on my own](https://github.com/hansemannn/titanium_mobile/commit/520dccdfef2ff82de635158977de442b17b9958c), but it did not seem to work.