Release wake lock when not moving

[mjaakko]

Currently the scanner service keeps a wake lock active even when not scanning. To reduce battery consumption, the wake lock could be released when scanning is paused and reacquired when scanning is started again. This should work at least when the significant motion sensor is used to detect movement as the sensor should be a wake-up sensor (i.e. works when a wake lock is not active): https://source.android.com/docs/core/interaction/sensors/sensor-types#significant_motion