Wifi Tx/Rx rate computation

Factor in the timestamp when computing the Tx/Rx rates. The
reported rate is a combination of current and previous rates
with a heavier weight from the current rate.

Bug: 30982272
Test: Ran the new image and compared the results against the
      old algorithm.

Change-Id: I485b7002416f382993353d9f6240ea63cb08ddae