ANDROID: net: xfrm: make PF_KEY SHA256 use RFC-compliant truncation. [v2]

When using the PF_KEY interface, SHA-256 hashes are hardcoded to
use 96-bit truncation. This is a violation of RFC4868, which
specifies 128-bit truncation, but will not be fixed upstream due
to backwards compatibility concerns and because the PF_KEY
interface is deprecated in favour of netlink XFRM (which allows
the app to specify an arbitrary truncation length).

Change the hardcoded truncation length from 96 to 128 so that
PF_KEY apps such as racoon will work with standards-compliant VPN
servers.

Note: this is effectively a redo (revert/reapplication) of:
  commit b8a72fd7
  ANDROID: net: xfrm: make PF_KEY SHA256 use RFC-compliant truncation.
but in a way that makes clearer what the ANDROID diff is.

Bug: 34114242
Bug: 120440497
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I8ee2ac9bb577b5078e8565a5b1f5fd84c2b3f74f