[pm] remove old stage dirs on low storage

As per suggestion 2 in https://b.corp.google.com/issues/67862680#comment16

Automatically delete stage dirs if the sessions are older than 8 hours.

BUG: 67862680
Test: builds
Change-Id: Ib1b9caf0165bfbe902feab5befd1213bd8428090
2 files changed