WifiInfo: Add getters for success tx/rx rate

This adds getters to get success tx/rx rate, returning in the
unit of 'packets per second'.
This also adjusts the constant OUTPUT_SCALE_FACTOR so it can
be more informative.

Bug: 62428282
Test: compile, unit tests
Change-Id: I989defd8fdc14dfc78692385fda86fb20b369b7b
1 file changed