The signature size will be 512 bytes when signing the payload
with 4096 bits key. This cl determines the key size with
"openssl rsa -modulus"
The new key in testdata is generated by
"openssl genrsa -out testkey 4096"
Bug: 129163830
Test: generate and verify an OTA package
Change-Id: I6662b0a0c553dc0fd84711312a1256b887e332fd