releasetools: Fix the path to the OTA keys in recovery image.

The recovery image will be packed under BOOT/RAMDISK only if
system_root_image and recovery_as_boot both are true (e.g. non-A/B
devices launched since P).

Bug: 113191245
Test: Run sign_target_files_apks.py on a target_files file that uses
      system-as-root but not recovery-as-boot.
Change-Id: I262a268055c6b5078d21694b5094a1c393d0d37c