Wifi usability: Change the unit of the threshold for tx/rx throughput from Mbps to Kbps

Data stall event can be triggered if the tx/rx tput is consecutively below a threshold over multiple RSSI polls. Changing the unit of the threshold from Mbps to Kbps allows us to configure a value below 1Mbps, e.g., 400Kbps.

Bug: 141027476

Test: frameworks/base/wifi/tests/runtests.sh

Signed-off-by: Mingguang Xu <mingguangxu@google.com>

Change-Id: Ib7b398fb6975c0d03b09550db9bf68a2bb8bddfb
Merged-In: I76d6338cd2d482d198fde1e5a2d1a0540c087ca6