Do not umount /vendor, /system, and /oem even if they are R/W.

- /vendor, /system, /oem can be remounted to R/W for development
  purpose.

- In such case, umounting these partitions can lead into some processes
  not running properly during shutdown or blocking umount of fs.

- So skip them. As it is dev feature, it is up to each developer to
  understand the risk. But for normal adb sync - reboot should be ok
  as shutdown involves sync operations.

bug: 37737296
Test: adb remount,reboot, and check last kmsg
Change-Id: Iab6a6374bc558375d359b3b49b14db93d363b1ad