Allow installing boot images outside of APEX.

After this change, `bootImageConfig.installDirOnDevice` can be set to a
path outside of the APEX, in which case, the boot image will not be
installed in the APEX. Instead, it will be installed to the given path
by Make.

This is a no-op change. Current behavior is not affected.

Bug: 211973309
Test: m nothing
Test: -
  1. m com.android.art
  2. See the boot image still being installed in the ART APEX.
Test: -
  1. Change `installDirOnDevice` of the ART boot image config to
     `system/framework`.
  2. See the boot image being installed in `/system/framework/<arch>`.
Change-Id: Ib13b17cc9e94dc5754c9b51b04df3307323b8783