Support removing AVB public keys

AVB public keys might be installed into the first-stage ramdisk.
This CL supports removing some AVB keys during the signing process,
which can be useful when a key needs to be revoked.

Also correcting the path of force_debuggable detection.

Bug: 150109393
Test: make dist
Test: ./build/tools/releasetools/sign_target_files_apks \
      --remove_avb_public_keys key1.avbpubkey,key2.avbpubkey \
      out/dist/*-target_files-*.zip signed-target_files.zip
Change-Id: I1af95adbbec655f40121f3392965dd1950aca4e5