Add property to skip idle state for zram writeback

Go devices benefit from a early writeback cycle of zram contents to
disk. Waiting for idle will delay the first writeback by a few hours.
Setting zram.force_writeback will allow kicking off writeback
when the device is not idle. The writeback takes a few seconds and did
not show noticible difference in user experience.

Bug: 141756630
Test: Device boots up and writeback works as expected
Change-Id: I4dd9a2d44f5ebd30f14ecbc6401dd05390c08bf6