Definition of the spatial coördinate system used used by Android

[Android Central Question]

Android uses a cartesian coördinate frame for representing vectors, such as the acceleration of the phone. However, relative to what is this frame defined?
For example, it makes sense to define a coördinate frame where some two orthogonal directions define the horizontal plane, while a third defines the vertical (upward direction). When Android defines one axis to be the vertical, how is this vertical defined? Is this vertical e.g. defined to be perpendicular to the Earth geoid (where it is assumed that the direction of gravity is in the vertical direction) or is it e.g. defined to be perpendicular to the Earth ellipsoid (where it is assumed that the model of Earth as we have it today returns the correct vertical for a certain location)?

Thanks a lot for your input!