Rename functions with a system/extras name collision.

Following around the call graph in code search is hard enough as it is!

Change-Id: I09d3513664423aafe0d99f9158acfbbb6c79b590
diff --git a/Ext4Crypt.h b/Ext4Crypt.h
index a363d39..a3d9224 100644
--- a/Ext4Crypt.h
+++ b/Ext4Crypt.h
@@ -36,9 +36,9 @@
                       char* value, size_t len);
 int e4crypt_set_field(const char* path, const char* fieldname,
                       const char* value);
-int e4crypt_set_user_crypto_policies(const char *path);
+int e4crypt_vold_set_user_crypto_policies(const char *path);
 
-int e4crypt_create_user_key(userid_t user_id, int serial, bool ephemeral);
+int e4crypt_vold_create_user_key(userid_t user_id, int serial, bool ephemeral);
 int e4crypt_destroy_user_key(userid_t user_id);
 
 int e4crypt_unlock_user_key(userid_t user_id, const char* token);