Only reset short term model after timeout

Here, we create a separate class, used to store the vital short term
model information; lux (aka, anchor), brightness and whether it is valid
or not.

When switching between interactive and idle screen brightness modes, we
save out the current short term model to the "mPausedShortTermModel" and
send an "invalidate" message, with a delay of the stm timeout.

If we do time out, then we invalidate the model, and if not, then we
will restore the model upon returning to the appropriate mode.

For now, the ShortTermModel class is within ABC, but could be moved to
BrightnessMappingStrategy later.

Bug: 269557114
Test: atest com.android.server.display
Change-Id: Ie3eed5a3acbb56f363f1ca8a61a6ecf64aae85c4