Set default EC key size to 256-bits

NIST SP800 131A recommends 112-bits as the lowest security level
acceptable after December 31, 2013. Although the 239-bit EC group
meets that bar, the P-256, P-384, and P-521 are the more widely
supported options. Change the default to increase interoperability
while maintaining the security level recommended.

(cherry picked from commit c5a7ff00b9c78ce5e15de5b99dc78a7e8c83ecd3)

Bug: 21085656
Change-Id: Idb71fdc801bafc5ad38f0b87dc3847f48854563f
2 files changed