adding support for low battery mode in the client side, which can override default service-side behaviour.
TODO actually take action on low battery.
Remove receiver in manifest for low battery notifications. The code
already checks this as part of scanning. Also, it is inflexible as it
notfies on a set battery level -instead, the code should handle a
parameterized battery level.
Send turn off broadcast when battery is low
Update with these changes:
- System ACTION_BATTERY_CHANGED intent drives the battery check code
- In the ClientStumblerService, in the battery checking code, if the
  battery is low and the user starts the service manually, battery
checking is deactivated until the battery is ok again.
- Send a dedicated low battery intent from the ClientStumblerService on
  low battery. (Received by MainApp)
Added 0% battery option
Merge pull request #1261 from garvankeeley/low-battery-mode

Low battery mode: make client-side configurable