Make __android_log_assert behave more like libc asserts.

If we don't output to stderr too, not only is it annoying to shell users (who
won't see anything), it prevents us from writing better gtests that actually
make assertions about the assert message.

Bug: http://b/23675822
Test: libutils tests still pass
Change-Id: I62b3144c385cba4dde485f0b0f9b42aeaef51e9a