Moved add_proposal() for SHA2_512

add_proposal() for SHA2_512 AES 256 breaks VPNs connecting to openSwan
servers. Support for SHA2_512 is broken on openSwan. To maintain backward
compatibility, we need to move add_proposal() for SHA2_512 below
SHA2_384 and SHA2_256. Verified with the fix on openswan, libreswan &
strongswan

Bug: 34755806
Bug: 34114242
Test: Verified on nyc-mr2-dev build
Change-Id: I71f5e09181e30137996a561676bf02dec07c30ef