Update ExtService to use suggestConversationActions API to generate replies

Also removed the SYS_PROP_SMART_REPLIES_EXPERIMENT checking, as we are
now hooking up to the real smart replies model.

BUG: 111437455
BUG: 111406942

Test: Sent a message to myself, observe the generated replies.

Change-Id: I521c5bf5ad010c462c5ea8e1ada565fdf9cab87d