fastbootd: sync() after flashing partitions or updating super

There is a chance that devices are failing to reboot in the lab due to
sync() taking explicitly long during reboot.  Let's add the sync()'s
here to ensure they get accounted for in the flashing process.

A side benefit is it's likely safer to sync immediately after
flashing than to hope init does it during reboot.

Bug: 150863651
Test: flash local devices successfully
Change-Id: I4c4b0114f3cde8af4b8b2cb283ec21f869ef9f6f