Reduce persist threshold for lower warning/limit.

Default is 2MB persist threshold, but even that can be substantial
for devices on 100MB/month plans. This change gradually reduces the
persist threshold up to 8x lower (256kb outstanding) based on lowest
active policy.

Bug: 5382676
Change-Id: Ief4e8cdb169bfb151a3d1b45722a8eaa01926508