Adjust minimum size of PKCS#1 RSA padding overhead.

This raises the space overhead estimate by one byte, due to me having
originally forgotten that when signing using RSA PKCS#1 padding
scheme, the plaintext of the RSA operation needs a leading 0 byte.

Bug: 21955742
Change-Id: If78efcd375afd8902a14a2217ddedfea87d4d337