Skip mount /oem on GSI

GSI self contains the full Android system part. Skip mounting
the oem partition on GSI, which is a part of system.

Bug: 158433698
Test: build aosp_arm64-userdebug.
Test: Check the file /system/system_ext/etc/init/config/skip_mount.cfg
Change-Id: I689e1cc8a68b980d68b1d6ef1d715203db078dc2