Also install verity_key to ramdisk for non-system-as-root target.

The commit in d14b8956
(https://android-review.googlesource.com/c/platform/build/+/728287)
changed partition layout, to always build the root dir into system.img,
even for devices not using system-as-root (i.e. the ones with separate
boot ramdisk).

With the new layout, there will be two root dirs for non-system-as-root
targets during the boot. If such a device uses Verified Boot 1.0,
/verity_key needs to be available in both roots, to establish the chain
of trust.
 - bootloader uses the baked-in key to verify boot.img; it then loads
   the ramdisk from the verified boot.img
 - First stage init uses /verity_key (in ramdisk) to verify and mount
   system.img at /system, then chroot's to it
 - Second stage init uses /verity_key (in system.img) to verify and
   mount other partitions

This CL adds rules to additionally install verity_key into ramdisk for
such targets.

Bug: 139770257
Test: Set up a target to use non-system-as-root
      (BOARD_BUILD_SYSTEM_ROOT_IMAGE != true). `m dist`.
Test: Check that both ROOT/verity_key and BOOT/RAMDISK/verity_key exist
      in the built target_files.zip.
Test: Run validate_target_files to validate the above target_files.zip.
      $ validate_target_files \
          --verity_key_mincrypt /path/to/verity_key \
          target_files.zip
Test: Run sign_target_files_apks to sign the above target. Re-run
      validate_target_files on the signed target_files.zip.
Test: python -m unittest test_validate_target_files
Change-Id: Ibe7e771c8c376429add85851ac86055564765d3c