mempress: change default PSI window duration to 2s

mempress: change default PSI window duration to 2s

This changes the PSI window duration we default to for watching memory
pressure events from 1s to 2s. This is because apparently the kernel
will soon disallow window durations other than 2s for unprivileged
processes.

Hence, we'll bump the threshold from 100m to 200ms, and the window from
1s to 2s.