There's no danger in plugging a USB device into a charger with a higher amp rating than the one supplied with it.
For mobile devices that use USB chargers, the difference between using chargers with different amp ratings in most cases is that the device will charge faster or slower, in direct proportion the the amp rating. In some cases, the device may simply draw less current than the charger's amp rating. In no case (except a malfunction) can the charger deliver more current than the device can handle.
The underlying issue to understand here is that the current draw is not an intrinsic property of either the power source or the connected device. It is a function of the voltage of the power source and the resistance of the device, per Ohm's law, which states
current = potential / resistance, or in unit terms,
amps = volts / ohms.
The current rating of a USB charger is the maximum current it can deliver*. If your device has a resistance of 5 ohms, and you connect a 5 volt charger, the device it will draw 1 amp of current. If the charger has a 2.1 amp rating, that means that it can deliver up to 2.1 amps, to a device that has a resistance of 2.38 ohms or less, but there is no way it can force 2.1 amps on a device with higher resistance.
The only time you need to worry about damaging the device is if the voltage of the power source is too high, in which case it can deliver more current than the device's circuitry can handle (see the footnote). However, the voltage of USB chargers is standardized (at 5 ±.25 V), so you never have to worry about damaging a USB device by plugging in a cable of the wrong specs.
However, most mobile devices have a low enough resistance to draw a 2.1 amp current. Most likely, Sony supplies 850 mA chargers because they're cheaper, not because that's the maximum current the device can draw (at 5 volts). I think you'll find that if you use a 2.1 A charger, it will charge the device more than twice as fast as the original charger.
* The amp rating of a device, if there is one, means something a little different. It's the current at which the device operates best, and the current that it will draw if connected to an outlet of the right voltage.
For example, if you have a vacuum cleaner rated at 12 A, that doesn't mean that it's a property of its circuitry that it "sucks out" a 12 A current from the outlet. It means that it should operate at 12 A, and will draw a 12 A current from the kind of outlet it's intended for. So, a 12 A vacuum cleaner sold in the US has a resistance of 10 Ω, and draws 12 A if plugged into a 120 V outlet. Take that same 12 A rated vacuum cleaner and plug it into a 240 V outlet, and it will draw twice the voltage, quadruple the wattage, burn out, and possibly start an electrical fire. On the other hand, if the voltage is too low, the vacuum cleaner won't run at full power, or won't run at all.