Replace RC4 in examples with AES-128-GCM.

Avoiding superflous references to RC4 makes it easier to audit the code
to find unsafe uses of it. It also avoids subtly encouraging users to
choose RC4 instead of a better alternative.

Change-Id: Ia27d7f4cd465e143d30a28b36c7871f7c30411ea
Reviewed-on: https://boringssl-review.googlesource.com/3990
Reviewed-by: Adam Langley <agl@google.com>