Add signing to keystore

Change the keystore to keep the private keys in keystore. When returned,
it uses the OpenSSL representation of the key to allow users to use it
in various operations through the OpenSSL ENGINE that connects to
keystore.

Change-Id: I3681f98cb2ec49ffc4a49f3821909313b4ab5735
diff --git a/keystore/java/android/security/IKeyChainService.aidl b/keystore/java/android/security/IKeyChainService.aidl
index f38f6ce..60fd7f7 100644
--- a/keystore/java/android/security/IKeyChainService.aidl
+++ b/keystore/java/android/security/IKeyChainService.aidl
@@ -23,7 +23,7 @@
  */
 interface IKeyChainService {
     // APIs used by KeyChain
-    byte[] getPrivateKey(String alias);
+    String requestPrivateKey(String alias);
     byte[] getCertificate(String alias);
 
     // APIs used by CertInstaller