Make Rollback lifetime duration configurable

Rollback lifetime duration is now configurable using a p/h flag.
The default remains 48 hours, and won't change unless another value is
pushed from the server.

The first expiration check now occurs after boot phase is done
instead of on service start. This neeeded to be done since Settings
Provider is not available on service start.

Test: atest RollbackTest
Test: adb shell device_config put rollback_boot
rollback_lifetime_in_millis 3600 and checked that the new value is 3600
Bug: 112431924
Change-Id: I6c28d9e945f28e340aff4914bfc772147667df2c