Add support for auth-only transforms

Kernel limitations prevent auth-only SAs from being created. Explicitly
request a null encryption algorithm instead of omitting the algorithm
to comply with the kernel requirement for ESP.

Bug: 75049573
Test: CTS tests for auth-only, crypt-only transforms added for all
combinations of (UDP, TCP, IPv4, IPv6, UDP-encap) Also added unit
tests to ensure correct triggering of NULL_CRYPT usage.
Change-Id: Ia9a5cfee9c7786412846bc039f326420f6211c08