Revert "Merge libdexfile_external into libdexfile." am: f442a84ca0 am: 4a0512c87a am: 6413358308

Original change: https://android-review.googlesource.com/c/platform/art/+/1660248

Change-Id: Ia93a20bf7527768f7e6b3aa94104fe8ee5fa3011