Use soong defined systemimage's avb_key for system_other

Bug: 376539388
Test: lunch aosp_cf_x86_64_phone-trunk_staging-userdebug
Test: Built make's and soong's target_files.zip and verified that
system_other.img is bit-identical

Change-Id: I3108e789849ba53f0b6e091fd387977722f4e01a