Move emulator fstab to vendor for "system as root"

In P, the root of the system image is used as the root of the
device, so all device (or vendor) spicific files need to be
moved to vendor partition. Relevant configuration files need to
be modified accordingly.

Bug: 76037265
Test: m -j; emulator # booted to home screen and
                     # checked fstab.ranchu is under vendor/etc

Change-Id: I2d2823407fcd792e3c1e24298ece466ef7bdb91f
Merged-In: I2d2823407fcd792e3c1e24298ece466ef7bdb91f
(cherry picked from commit b94bc998e2712210fc6118fb44a79f001581441d)
diff --git a/init.ranchu.rc b/init.ranchu.rc
index de5c583..ff3abae 100644
--- a/init.ranchu.rc
+++ b/init.ranchu.rc
@@ -1,5 +1,5 @@
 on fs
-    mount_all /fstab.ranchu
+    mount_all /vendor/etc/fstab.ranchu
 
 on early-init
     mount debugfs debugfs /sys/kernel/debug mode=755