What is the audio voltage of an android phone?
There isn't really a standard, it depends on exactly what headphone amplifier the phone uses. The closest thing is the -10dBV (.9VPP) consumer line level output, but I doubt most phones have the actual output level calibrated to that standard. It's more than likely that they tested it by plugging the phone into a couple of AV receivers and found that it got "loud enough" with some volume adjustments. 

In any case, the headphone amplifier's job is to push current through a load which might have a wide range of impedances. This means that the maximum output voltage into a high-Z load isn't that important for the intended application, and is sometimes omitted from the datasheet. Your best bet is to take apart or look at disassembled photos of various phones, figure out what headphone amp they are using, and go from there. For example, the TI TPA6166A2 quotes:

VOUT,Max Max line output voltage RL = 10 kΩ 1 VRMS

But many other devices omit this spec. You can probably estimate it from looking at the power output and using the average power equation with the quoted load.