implement service as required for notification
use service to wrap around udp sender task

Pressing the notification creates a new activity, hence the need to save
settings when starting.
unbind from service when application stops as per api
acquire wifi and cpu wakelocks in sender
pointless to conditionalize reset of condition variable
synchronize sensor event/send, don't crash with gyro lackage

Sensor event handler and send both modify member variables, and run
asynchronously. Both blocks need synchronization as result.

Gyro-less data is mostly useless, but it's not necessary to crash on
such devices in any case.
don't force screen on as poor wakelock substitute