power: Ambient light moving average now time-aware

The readings from the ambient light sensor were being filtered using an
exponential moving average (equivalently, a first-order low pass
filter), but the average did not account for the time since the previous
reading.  Now the power manager records the time of the last reading,
making the brightness adjustments more even.
6 jobs for als-filter in 2 minutes and 48 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #807399
build

00:00:40

 
  Test
passed #807400
test

00:02:07

 
  Manual
manual #807401
privileged allowed to fail manual
asan
manual #807404
allowed to fail manual
msan
manual #807402
allowed to fail manual
tsan
manual #807403
allowed to fail manual
ubsan