Move recovery building from sign_target_files_apk to add_img_to_target_files

Currently we're building the boot/recovery image twice, which is
redundant. And b/38455129 shows a problematic case when the image
from two builds doesn't match. We should only build the recovery
image once in the add_img_to_target_files.

Bug: 62021378
Test: call sign_target_files_apk on an angler target file,
recovery-from-boot.p generates successfully; and SHA of recovery.img
matches the one in install-recovery.sh.

Change-Id: I01e033501d80c18a87cbb870300eee5c19a04441