Reland: Moving GSI-specific files to system_ext

Only common files can reside in system partition, other files
should be moved to the newly added system_ext partition.

Note that for GSI, it will be a single system.img that includes the
contents of product and system_ext partitions, under /system/product
and /system/system_ext, respectively. After moving skip_mount.cfg to
system_ext partition, it also needs a symlink file under system
partition:

  /system/etc/init/config -> /system/system_ext/etc/init/config

This allows Q-launched first-stage init (in /boot partition) continue
to use the same path when new GSI image is used.

Bug: 138281441
Test: build aosp_arm64-userdebug and boot it on crosshatch
Test: rm -rf out && build/soong/soong_ui.bash --make-mode \
      TARGET_PRODUCT=aosp_arm64 TARGET_BUILD_VARIANT=userdebug droid
Change-Id: Iae9f5fb688f49497563864eb882d5f0ae33c744a