Grab wakelock when we may become postured soon.

If the latest event in the sliding window has a higher confidence then
the threshold, then assuming no new events come in, we will eventually
become postured. Therefore we can use this as a signal to know that we
may be postured in the near future, and grab a wakelock to keep the
device awake.

This helps ensure that the device doesn't go to sleep before we have a
chance to start dreaming.

Fixes: 409896203
Test: atest PosturingInteractorTest
Flag: android.service.dreams.allow_dream_when_postured
Change-Id: Id0b39ceec5d756b4c9fb10e7ca1b722cb9fc45d5