Add support for always-on option

For power-consumption reasons, a platform may choose to throttle or even suspend sensors so that the handset can reduce its power consumption.
Even if the platform stops sensors when the display is off, apps should be able to set a property to keep the sensor running while the display is off.

This could potentially be standardized at the sensors API level.

ie.

• default = platform may slow or stop sensor
• set a property to run sensor with the display off