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

core/java/android/net/IpSecAlgorithm.java

@@ -37,6 +37,13 @@ import java.util.Arrays;
 public final class IpSecAlgorithm implements Parcelable {
     private static final String TAG = "IpSecAlgorithm";
 
+    /**
+     * Null cipher.
+     *
+     * @hide
+     */
+    public static final String CRYPT_NULL = "ecb(cipher_null)";
+
     /**
      * AES-CBC Encryption/Ciphering Algorithm.
      *