Delete cached A-GPS aiding data to force GPS cold start

Issue #9

According to "Professional Android Sensor Programming" and cargo cult programming advice on Stack Overflow, the device's cached A-GPS data can be up to 5 minutes out of date. The `delete_aiding_data` and `force_extra_injection` GPS commands will force the device to download the latest GPS ephemeris and almanac data. The `force_time_injection` command will resynchronize the system clock with NTP.