Do not umount roofs even if it is R/W.

Latest device has rootfs instead of "/system" mount point

(cherry picked from commit a01c27eef8e9361df63a62c916619fb809bc647b)

Bug: 37737296
Test: adb remount, reboot, and check log
Change-Id: I315ecf71e85255fc55c3a80619920b456bad0956
1 file changed