Add "Pulse low battery light" switch.

Implements the framework portion of an optional "Pulse low battery light"
switch, which allows the pulsing low battery LED to be disabled.  The
function and implementation of this switch is analogous to (and borrowed
from) the "Pulse notification light" switch.

This is motivated by devices (e.g., epicmtd) featuring a battery charging
LED, where the LED does not support pulsing in hardware and relies on a
kernel (CPU) timer to implement the pulse capability.  On these devices,
pulsing the LED requires waking the CPU from suspend on every LED on/off
event, substantially reducing the proportion of time the CPU can suspend,
and thus, substantially increasing power consumption.

Therefore, when the these devices reach the low battery threshold, their
power consumption, ironically, increases measurably in order to pulse the
battery charging LED, which lowers battery life and increases the
likelihood of a low-battery shutdown if the user cannot charge the device
in a timely fashion.  This switch allows the user to disable the pulsing
low battery LED in order to maximize their low battery charge.

Change-Id: I3a5c8938fd7c444c90b2af123e683982c614cd6e