Convert -dB to equivalent +dB
Decibels are always defined relative to some reference. For instance, 0 dB corresponds to 1 x reference, and 20 dB corresponds to 10 x reference (using your formula). Your Android sound setting is simply a normalised value between 0 and 1, with 0 being completely silent and 1 being the maximum output the speakers can handle, and it has nothing to do with dB, positive or negative.

The problem is that you don't know what 0 dB (i.e. 1 x reference) corresponds to on that scale. I guess you could try to find out using the Android developer documentation, but I bet this value is just arbitrary and probably varies from manufacturer to manufacturer, so even if you find out for one phone it'll probably not work for others.

I suppose a way to fix this might be to first get the user to calibrate the reference by first recording the ambient background noise in the room they are in, which will allow you to set 0 dB and then output sound volumes with respect to that.