are the accelerometers on Android always on? or is it possible to enable/disable them? Does by chance activating/deactivating the setting "Auto-rotate screen" by chance activates/deactivate the accelerometer?
The accelerometers are hardware, thus always ON. When you deactive the settings that need them (your example Auto-rotate screen is a good one), you merely deactivate the reading of that sensor to control the activity. A good proof of this is to use the game app "Drop", which uses these sensors. While you may turn off Auto-Rotate, the sensors are still live for the game.