Create a Zram writeback job

Zram on some devices can support writing idle pages
on to disk. ZramWriteback schedules jobs to track idle
pages and trigger write-to-disk when the device is idle.

Note: This is the same change that was reviewed in aosp. I am doing
a manual merge to avoid automerger merge conflicts.

Bug: 117682284
Bug: 122674343
Test: dumpsys jobscheduler
No jobs should be scheduled for devices with no support for
zram writeback or if they do not have low_ram flag set.

Change-Id: Id1bc41ac32f2c63e241270c10a07560bda39a127