[batterytracker] Skip battery reading on first reading after boot

statefs is giving wrong battery info on first reading.
We skip that and try again after one minute.
It is ok to delay lowbattery shutdown by one minute if battery is actually low. User gets change to see reason for shutdown. If dsme would do shutdown right after boot user would not see UI at all and have no clue why boot ended to shutdown.

Signed-off-by: Pekka Lundstrom <pekka.lundstrom@jollamobile.com>