Display OMEMO device ID

Display the OMEMO device ID which has been generated, when the user generated
OMEMO crytographic materials via /omemo gen.

tests/unittests/omemo/stub_omemo.c

@@ -114,3 +114,8 @@ omemo_encrypt_file(FILE* in, FILE* out, off_t file_size, int* gcry_res)
     return NULL;
 };
 void omemo_free(void* a){};
+
+uint32_t
+omemo_device_id() {
+	return 123;
+}