Addressing error-code propagation for remount service.

Bug: 201596822

Test: Local test (isolated to host)

$ adb -s 8BDAY00CM6 root
adbd is already running as root
$ adb -s 8BDAY00CM6 remount
Disabling verity for /system
Using overlayfs for /system
Disabling verity for /system_ext
Using overlayfs for /system_ext
Disabling verity for /vendor
Using overlayfs for /vendor
Disabling verity for /product
Using overlayfs for /product
remount succeeded.. now reboot device for settings refresh
$ echo $?
0
$ adb -s 8BDAY00CM6 unroot
restarting adbd as non root
$ adb -s 8BDAY00CM6 remount
Not running as root. Try "adb root" first.
remount failed
$ echo $?
3

Signed-off-by: Shaju Mathew <shaju@google.com>
Change-Id: I8e6d0735d2da0ed58cae867db2e4736abb86d351