Use stream operator to report result code

The cast operator for KeyStoreNativeReturnCode and
KeyStoreNativeReturnCode will be removed later.
There already exists a stream operator to get
at the underlying value. Use that instead.

Bug: 119771891
Test: Built for walleye successfully, basic operations with
      keystore_cli_v2 tool work correctly.
Change-Id: I357e0582841b1deadbffd59bb49b5ae8542c9aee
1 file changed