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: I88bfdbc1d7791f6afb64edf352b4898a50b41190