ab8500-fg: Adjust for RF bursts voltage drops

Changed conditions for restarting low battery measurements counter
and adjusted the interval between measurements to avoid RF burst
induced voltage drops, and to shorten time to decide to shut down.

Signed-off-by: Hakan Berg <hakan.berg@stericsson.com>
Signed-off-by: Martin Bergstrom <martin.bergstrom@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Jonas ABERG <jonas.aberg@stericsson.com>
Reviewed-by: Marcus COOPER <marcus.xm.cooper@stericsson.com>