Work around an Android SSLEngine issue

Motivation:

Some Android SSLEngine implementations skip FINISHED handshake status
and go straightly into NOT_HANDSHAKING.  This behavior blocks SslHandler
from notifying its handshakeFuture, because we do the notification when
SSLEngine enters the FINISHED state.

Modification:

When the current handshake state is NOT_HANDSHAKING and the
handshakeFuture is not fulfilled yet, treat NOT_HANDSHAKING as FINISHED.

Result:

Better Android compatibility - fixes #1823