pixel3_mainline: device.mk: make fstab device-specific am: 37a07c0917 am: 67033e221a

Original change: https://android-review.googlesource.com/c/device/linaro/dragonboard/+/1723134

Change-Id: I48b5b5e1cc8b2f0bf0d9c3aaeecaf9f5a40b91a2