Android zram writeback

commit message:

"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."

the functionality is based on idle page writeback support, and requires CONFIG_ZRAM_WRITEBACK=y in kernel config. losetup can be used to create loop device from swapfile, so backing_dev could be assigned to.

it also requires config_zramWriteback to be set enabled.

the commit introduced three properties (default value from pixel).
ro.zram.mark_idle_delay_mins = 60
ro.zram.first_wb_delay_mins = 1440
ro.zram.periodic_wb_delay_hours = 24

so it works like:
add timer do (run every periodic_wb_delay_hours * 60 then fork run)
sleep(first_wb_delay_mins)
:run
write all to zram devices(mark as idle)
sleep(mark_idle_delay_mins)
start writeback

ref:
https://www.kernel.org/doc/html/latest/admin-guide/blockdev/zram.html
services/core/java/com/android/server/ZramWriteback.java from android platform/frameworks/base