system: core: increase the polling time for remount_ro

eMMC4.5 introduces an addition of cache on the device.
We experienced long flush durations in some of the devices,
and therefore the adb reboot resets the target before the flush
completes.

CRs-Fixed: 562350
Change-Id: I3f54901e12ffd232a7696dadfde1edec99ef91c6