Fix (pvmfw.img) Embed correct public key for Microdroid verification

The previous commit (I551e14fa6a0c63e3cef334b953f670cf9c465e10)
incorrectly embedded the APEX public key ('apex_pubkey') into
pvmfw.img. This key is used to verify `apex_payload.img`
within `com.android.virt.apex`, not the Microdroid image.

This commit embeds the correct public key, which verifies
`microdroid_vbmeta.img` inside `apex_payload.img`.

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: unzip signed.zip IMAGES/pvmfw.img
Test: avbtool extract_public_key --key external/avb/test/data/testkey_rsa4096.pem --out key.pub
Test: grep -U -F -f key.pub IMAGES/pvmfw.img => grep: IMAGES/pvmfw.img: binary file matches
Change-Id: Ic8ae72898b8ab6067402b26eef9ed1b876a778f7