Clarify the architecture diagram

[aaaaalbert]

• The box currently reading "Sensors" really combines multiple different data sources, including what Android calls sensors. This should be three boxes, one containing the second containing the third, with the innermost being a subset of Android sensors available through NDK, the middle being all Android sensors, and the outmost being additional sources such as GPS, the battery meter, and WiFi (aka what Sensorium and Sensibility call "sensors").
• Not all of the boxes exist in all scenarios. For example, the "code samples" diagram has no Python/Repy/user code. The JVM/DVM does not exist in "aaaaalbert/snakei" (unfortunately!). Consequently, ....
• Write accompanying text explaining why which parts are missing, and what the subsequent difficulties (if any) are. E.g., sample code wraps all the native stuff in a thin app layer, and thus ensures that Android launches a DVM so that JNI works. We can't currently do that (at least not with unmodified Python), so we get no DVM.