Moving skip_mount.cfg to system_ext partition

We're now adding a core CSI system.img that is common across different
targets. So GSI-specific files should be moved to system_ext partition.

Note that GSI is a system.img that includes the contents of product and
system_ext partitions, under /system/product and /system/system_ext,
respectively. That's why it's under /system/system_ext instead of
/system or /system_ext.

Bug: 137711197
Test: boot GSI on crosshatch
Change-Id: I1ee54c1a434f2d7cf34b4ea2cbda9c732ec932a1