How to save as 32bit PNG - how to convert the default transparency layer into an alpha layer

For android, you have to supply a 32bit PNG.  See this link: Status Bar Icons  |  Android Developers

I have created my icon with two layers, a white layer and underneath a transparent layer.

I understand that a 32 bit PNG means 8 bits each for R, G, B and Alpha.

I added an alpha channel under "channels" tab thing. But I dont think this helps, as the transparent layer I have is nothing to do with alpha.

If I go to file - > export as, I can chose a PNG. I seem to remember in the old days there was an option export as PNG32, but this seems to have gone.

So the question is:

1) how do I convert a normal photoshop image into something I can safe as PNG32 (e.g. how to I convert to use alpha for transparency)?

2) how do I actually save it so that Android can use it?

I have heard that GIMP has better tools for saving as PNG32, should I switch to this for this type of image?