system: core: increase the polling time for remount_ro to 3600

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.
Latest tests show that increasing it to 2400 wasn't enough thus
it was increased to 3600.

Change-Id: Ifbe7af16aa1861d0d57c808b0240361c0317d5c5
CRs-fixed: 581363