Support relative key path in VerifiedBootVersion2VerityImageBuilder

Otatools archive can be unpacked to an arbitrary directory together
with bundled external/avb/test/data keys. Calling add_img_to_target_files
from a different directory fails to build an image which has
avb_{}_key_path pointing to the bundled key since add_hash_footer
can't find the key file.

Fix this by applying the same logic as in common.AppendAVBSigningArgs.

Steps to reproduce:
$ . build/envsetup.sh && lunch aosp_cf_arm64_phone-userdebug && make dist
$ cd out/dist && rm -rf otatools && unzip otatools.zip -d otatools
$ zip -d aosp_cf_arm64_phone-*target_files*.zip IMAGES/system_other.img
$ otatools/bin/add_img_to_target_files -a -v -p otatools \
    aosp_cf_arm64_phone-target_files-*.zip

Bug: 218531463
Test: manual, see "steps to reproduce" above
Change-Id: Id3f7cf2678e998ef1beda2b6741944c6455c2404