Pass the correct signature size to payload generator

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