how do i create a custom interpolator for android animations?

This is done by making a class that implements Interpolator. Or you can use a SAM-converted lambda. For example, an interpolation based on the smoothstep function:

animationSlide.setInterpolator { amount ->
    amount * amount * (3f - 2f * amount)
}

If you want to define a set of points to define your curve, you can do that with the Path class and instantiate a PathInterpolator with that. From looking at the source code, it looks like PathInterpolator approximates the Path by breaking it down into straight line segments, so if there are arcs and Beziers, it could potentially become kind of a heavy class instance that you might want to cache a reference to if you use it repeatedly.