Fix the signing error in gsi_arm64 builds

After adding 'PREBUILT_IMAGES/pvmfw.img' into gsi_arm64,
the signing process fails:

common.ExternalError: Failed to run command
  '['avbtool', 'extract_public_key', '--key', 'PRESIGNED',
    '--output', '/tmp/avb-8z8y8_xn.avbpubkey']' (exit code 1):
  ~/codebase/android15-tests-dev/otatools/bin/avbtool:
    Error getting public key: b'Could not open file or uri for loading
    private key of public key from PRESIGNED: No such file or directory\n'

This is because that apex files are pre-signed in gsi_arm64
and the script currently tries to extract public key from the
non-existing 'PRESIGNED' file.

Fix this by obtaining the public key from 'apex_pubkey' of
'SYSTEM/apex/com.android.virt.apex'.

See https://source.android.com/docs/core/ota/apex#apex-format
for details.

Bug: 384813199
Test: m sign_target_files_apks
Test: sign_target_files_apks --allow_gsi_debug_sepolicy \
        --extra_apex_payload_key com.android.virt.apex= \
        -e com.android.virt.apex= \
        gsi_arm64-target_files-${build_id}.zip signed.zip
Test: `zipinfo signed.zip | grep pvmfw`, checks pvmfw.img is included.
Change-Id: I551e14fa6a0c63e3cef334b953f670cf9c465e10