Allow installing boot images outside of APEX for prebuilt.

This change is similar to aosp/1947127, but for prebuilts.

After this change, if `bootImageConfig.installDirOnDevice` is set to a
path outside of the APEX, the build system will build a boot image from
the dex files and the profile extracted from the prebuilt APEX.
Otherwise, it keeps the current behavior: extracting the boot image from
the prebuilt APEX.

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

Bug: 211973309
Test: m nothing
Test: -
  On internal master:
  1. Patch aosp/1947128.
  2. Patch ag/16743847 and ag/16746804.
  3. m SOONG_CONFIG_art_module_source_build=false com.google.android.art
  4. See the boot image being installed in `/system/framework/<arch>`.
Change-Id: I24ca525309fecaf3ab7a67960fbf118cd00ecd1d