Rather than using hard-coded system, use ANDROID_ROOT
environment variable to allow AB devices to mount
system_root at a custom path. This allows the /system
path to be bind mounted from $ANDROID_ROOT/system
so that the vold_decrypt paths can remain unchanged.
Change-Id: I9a7b13385e43f169f1df4c75b2a003fc6913952c
6ac30afd69