Update a couple of blobstore params to be configurable.

Instead of hardcoding values for IDLE_JOB_PERIOD_MS and
SESSION_EXPIRY_TIMEOUT_MS, make them configurable.

Bug: 157503601
Test: atest --test-mapping apex/blobstore
Change-Id: I4846a98144224873444d4d73b2bedde258992ac4
6 files changed