Optimize _apkcerts_write_line

Emit 1 echo command per line instead of 2-4. The build was failing
due to the large number of commands, although I'm surprised because I
thought there was no limit on the size of the commands if they were
in a bash script file.

Bug: 378007883
Test: m out/target/product/vsoc_x86_64_only/obj/PACKAGING/apkcerts_intermediates/aosp_cf_x86_64_only_phone-apkcerts.txt
Change-Id: I29ee8a80c07e47db05e1bb3189a9786425560e9f