Roboto is the standard typefaces on Android according to the Material Design Guidelines. As a general rule, Android products designed for Nets should follow this rule as well.
Typograhpy
Typographic scale
The Material Design guidelines suggest using a typographic scale in order to avoid having too many type sizes and styles at once in your design. The use typographic scale offers a limited set of type sizes that work well together along with the layout grid.
These sizes and styles were developed to balance content density and reading comfort under typical usage conditions. Type sizes are specified with sp (scaleable pixels) to enable large type modes for accessibility.
The basic set of styles are based on a typographic scale of 12, 14, 16, 20, and 34.
Please refer to the Typography Style page from the Material Design Guidelines where you can find more details about the typographic scale.
More information
Find out more about typography in your Android app by accesing the dedicated page of Material Design Guidelines.