Using a USB 3 port as a power delivery port for quicker charging of connected devices
You are right. USB3 could do that, but needs a partner USB of same family to do it.

Thus the USB2 would be the bottleneck. 

However the USB2 already can handle up to maximum of 900mW. 


  Devices can request higher currents and supply voltages from compliant
  hosts.


In simple way, if your Android makes the correct request it could get up to 900mW supplied. You can check how much it gets in the About this Mac.

read more here https://electronics.stackexchange.com/a/5581/77190

and here:https://superuser.com/q/293189