Support verifying system_other

This commit extracts the AVB key used to sign system_other.img into
system.img, for init to verify system_other's AVB metadata.

The extracted key will locate in:
    /system/etc/security/avb/system_other.avbpubkey

Bug: 123611926
Test: build and checks the following is generated
      $OUT/system/etc/security/avb/system_other.avbpubkey

Change-Id: Icdc703ff5a0d50f8140bb652507b9b4cbc8a2118