I would doubt that, and I would also doubt that you can simply define opening an app as the beginning of a legally binding, entirely new contract with as many implications as Uber's driver contracts have. I am pretty sure that in reality, Uber signs a contract once with a new driver, after checking his background, driver license and everything, and that contract is active from that point in time until explicitly terminated by either party. Just closing the app does not terminate it, neither does opening the app create a new contract. Closing the app just amounts to the driver leaving his "workplace", opening it again amounts to the driver entering his "workplace", but the contract under which that work takes place is the same and active all the time, it just gives the driver a lot of leeway with regard to how many hours he wants to work.
That is a generous employment, but it is an employment nevertheless in nature.
When the driver activates uber driver mode, they are initiating a contract which states they will work within the bounds of the app. The driver is able to terminate that contract at will and at any time, and reactivate it at will and at his leisure. When the contract is active, the driver has committed to operate within the pricing, pickup and dropoff structure of the platform - as in the driver has committed to minimize discrimination of their rides and accept the platform's rider oriented pricing structure.