Wifi usability: Add firmware alert reason code into WifiUsabilityStats

Firmware alert can trigger Wifi usability stats data collection.
Currently the reason code of firmware alert is not collected as an
attribute.  Add the reason code into Wifi usability stats as an
attribute.

Bug: 129426122
Test: frameworks/base/wifi/tests/runtests.sh

Change-Id: I5640745b5ee4588bb8ac106420262685ad8d6a28
Signed-off-by: Mingguang Xu <mingguangxu@google.com>