How does an oscilloscope buffer work?
You can't sample continuously.
However, this isn't too much of an issue usually with scopes as most don't do this (at least at the higher speeds) Even an analogue scope does not display continuous information as you have the beam sweep back period.  

A typical digital storage scope waits for a trigger, fills it's buffer at high speed, then sends the data at a lower speed to the microcontroller (or android in your case) for display. As Photon mentions, this happens fast enough (e.g. above 15 frames of data a second) for it to look continuous to the user.  

Obviously the larger RAM buffer, the more samples(time) can be captured. This is why you will see DSOs advertising large capture buffers. At high speed this can make a big difference - at 2 Gigasamples per second and a 10KB capture buffer you are only going to get 5us of information. Some with larger buffers can only use part of it at the highest speed - for instance my DSO has 10MB of buffer up to 1 Gsps, but can only use 10KB at 2Gsps.    

In your case you could switch to continuous capture at speeds the USB is capable of (e.g. &lt; 1 Msps) You could maybe look at using compression also.
Either that or look at using high/super speed USB.