Curve 25519: Support use via Android Keystore provider (part 1)

Ensure that the user gets an accurate error message when they try to
generate Curve 25519 keys according to JEP 324
(https://openjdk.java.net/jeps/324).

Android Keystore requires every key to have a name, so it is not
possible to generate a key using NamedParameterSpec only (with a
KeyPairGenerator).

Support this and throw an exception to the caller indicating how they
_can_ generate keys with this curve.

Bug: 222440855
Bug: 195309719
Bug: 194359292
Test: atest android.keystore.cts.KeyFactoryTest android.keystore.cts.Curve25519Test
Test: atest CtsLibcoreTestCases:libcore.java.security.ProviderTest
Change-Id: I5aa163f177507906c6482d079eb6cb55d93accf7