Don't invalidate short term model when idle

We don't want to invalidate the short term model when idle, as this
mode requires the short term model to remain active for the entire
duration of being idle.
We reset the short term model after, when switching from idle to
interactive.

Bug: 205686510
Test: manual
Change-Id: I10e66d86711024af64ab6a6afc5dd8d7b4117041