Li-Po Charging from automotive supply and powering load at 3.6v
There are battery managers with Buck converters to give you what you want. You'll need to find one that fits well with your system, but here's an example: http://www.ti.com/product/BQ25703A

Integrating control with the level converter simplifies the power system. You'll want to be sure to shut down if the battery goes under voltage like the reference design schematic in the link shows.