Resign microdroid_vendor.img with avb_vendor_key

microdroid_vendor.img is vendor image for running Microdroid, a type of
VM run on top of Android. microdroid_vendor.img is currently stored in
host device's vendor partition. However the original key signed for
microdroid_vendor.img is not enrolled in the signing server, so it can
make possible test breakage if there's a test checking all files in the
partition is signed with trusted key.

Therefore, this is the patch to resign micrdoroid_vendor.img with
avb_vendor_key. When vendor image of host device is resigned with that
key, microdroid_vendor.img would be resigned as well with the same key.

Bug: 285855442
Test: First, for testing, modify the script to skip all files except
VENDOR/etc/avf/microdroid/microdroid_vendor.img in ProcessTargetFiles.
Second, run following commands and check if script doesn't throw any
error until ProcessTargetFiles ends.
- sign_target_files_apks --avb_vendor_key external/avb/test/data/testkey_rsa2048.pem --avb_vendor_algorithm SHA256_RSA2048 <source_zip_file> <target_zip_file>
- sign_target_files_apks --avb_vendor_key external/avb/test/data/testkey_rsa4096.pem --avb_vendor_algorithm SHA256_RSA4096 <source_zip_file> <target_zip_file>
- sign_target_files_apks --avb_vendor_key external/avb/test/data/testkey_rsa8192.pem --avb_vendor_algorithm SHA256_RSA8192 <source_zip_file> <target_zip_file>
Third, `unzip <target_zip_file>` and do `avbtool info_image`

Change-Id: I5337f61ab9eca7e6d0f92860486bc820b6e09eac