Keystore: Support Ed25519 keys

Implement support for Ed25519 signing keys in Android Keystore.
Because Conscrypt does not yet handle those keys, the Keystore classes
implement EdECPublicKey directly and parse the keys.

Specifically, AndroidKeyStoreEdECPublicKey can take an encoded X.509 key
specification, validate the encoding is of an Ed25519 key, then parse
the oddity and Y point on the curve.
RFC8032 describes EdDSA signature scheme, particularly Ed25519.
RFC8410, Section 3, defines the OID for Ed25519 keys (1.3.101.112).
RFC8410, Section 4, describes the encoding of the public key.

Bug: 195309719
Bug: 194359292
Bug: 214203951
Test: atest android.security.keystore2.AndroidKeyStoreEdECPublicKeyTest
Change-Id: I07b793cbd5029630768368ad4a863bbc1c828ced