The feature is already available, it's under the settings menu.
However uptime isn't the only factor to consider.
Most phones will not have enough processing power to throughput a significant amount of traffic.
Most network providers will be behind some kind of NAT'd connection and it would be difficult if not impossible to allow users to connect to your devices ORPort.
Phones will often move between different network types, opportunistically using available wireless connections. These transitions would make them unusable until the next consensus was published.
So while there is no reason a phone can't provide a relay, there are quite a few reasons that it likely wouldn't provide a very good or useful one.