How would one use the Android accelerometer to measure one's mass?
Newton's second law states that Force = Mass * Acceleration.  We can use this formula to solve for Mass:: Mass = Force / Acceleration.  As such, we need a known force in order to take a measurement of acceleration to determine mass.  Hopping into the air is going to be a variable, instantaneous force.  Also, your legs are going to act like variable force springs to absorb your impact with the ground.  All of these variables would really frustrate your attempt to get anything close to a valid mass reading.

NASA has created device that uses the measurement of acceleration to determine mass for astronauts in space.  The Space Linear Acceleration Mass Measurement Device (SLAMMD) uses two springs to generate a known force on the subject and then reads the acceleration to determine mass.  There is a nice video on YouTube explaining how it works and providing a demonstration of the device in action.