Revert "releasetools: Don't extract BOOTABLE_IMAGES when signing target files"

sign_target_files_apks is currently dropping the content of
BOOTABLE_IMAGES. As consequence, the images are re-generated
without using custom mkbootimg makefiles, which can cause
issues on some devices with specific formats or requirements.

This reverts commit 71fbe14d.

Change-Id: If4c8c70ee32e21198859f594a2dc090abe2e13a1
(cherry picked from commit 412260ec)