gapii/client: Track MEC in analytics events.
gapii/client: Track MEC in analytics events.

Add a "mec" label to the capture analytics event, if the capture is
an MEC capture.

google/gapid,pmuetschard/gapid,Qining/gapid,google/gapid,google/agi,Qining/gapid,google/gapid,google/gapid,google/agi,Qining/gapid,google/agi,pmuetschard/gapid,google/agi,Qining/gapid,google/gapid,pmuetschard/gapid,google/agi,google/agi,pmuetschard/gapid,google/gapid,pmuetschard/gapid,Qining/gapid,google/gapid,pmuetschard/gapid,pmuetschard/gapid,Qining/gapid,pmuetschard/gapid,google/agi,google/gapid,Qining/gapid,google/agi,Qining/gapid