Moving GSI-specific properties to /system_ext

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

Also renaming various generic*/system.prop to generic*/system_ext.prop.
This is to put the customization into /system_ext/build.prop instead of
/system/build.prop.

Bug: 137711197
Test: boot a GSI on crosshatch, and checks the value of those properties
Change-Id: Id344124280d5f4a6c10d390a9e8a4a50cc7f28fb