Take a look at this thread for an explanation of what constitutes a public performance. Just by operating your tablet for your personal use in public would not be performing the music or any other copyright material.
If you were deliberately using your tablet in order that others would be entertained, then I suppose technically that might amount to making the music available to the public which would require a licence from PRS/PPL, much like the sort of licence shops and similar outlets need in order to have the radio on in the public part of their premises. However I think it would be very hard to establish, for legal purposes, that that was what you intended and coincidentally, there just happened to be someone from PRS/PPL around at the time to notice!
Probably best to use earphones and that way you don't disturb others with your choice of music!