Sensing Mobile Battery full and cutting down power on V_bus line
If the mobile is just sleeping and wakes up on its own it will draw more current not going to charge the battery.  For this reason the responsibility of charging the mobile battery must be internal to the mobile and not as you would like.  The mobile will also be responsible for indicating when it is full.

So the problem has no need for a solution and no way to detect where the load current is going.. to power the mobile charger or the rest of the mobile loads.