Send battery percentage with GPS data

[eirmd]

This PR adds the ability for the Android app to include the device battery percentage with each GPS/location update sent to the server.

• Adds BatteryUtil.java to read battery percent (0–100)
• Includes battery value in the position payload
• No extra permissions are required
• Backward compatible: works with older server versions (battery ignored) and older apps (battery stored as NULL)