Don't defer custom keepalive expiry on all payloads

pyghmi was using any activity to defer any keepalive.  If
a caller has a custom keepalive, only keepalive activity
should advance the keepalive expiry.  Modify code to defer
keepalive only if it is the generic keepalive.

Change-Id: I852ad7a5de65af60fb8e11580bd2ef32896b71f6