Google launched Android Design which provides design guidelines to achieve a consistent style across the platform. It includes information about:
General principles such as consistency ("If it looks the same, it should act the same") or use of unobtrusive notifications ("Only interrupt me if it's important").
Layout and visual style including icon design, typography (Roboto typeface), color scheme, themes, considerations for multiple screen sizes and densities.
Interaction patterns and specific widgets to support them. You can find design guidelines to resolve common problems (navigation, user notification, etc.) and widgets the platform offers (buttons, lists, text fields, etc.) with recommendations for their use. I have included below an image used to illustrate the design of notifications: