HFP: Open SLC after AT+BIND if HF indicators supported

If HF indicators is supported by HF and AG. Service Level Connection
shall consider established after the HF has successfully retrieved
information about HF indicators currently enabled by the AG using the
AT+BIND? command.

Bug: 169935138
Test: connect headset during a call
Tag: #compatibility
Change-Id: I67cfac3b035904a2375c77d726a8e16accf2b014