Fixing the BandwidthController unit test

The BandwidthController unit test cannot directly use gCtrl->trafficCtrl
to check if the bpf stats feature is avaliable. Rewrite the
getBpfStatsStatus() function of BandwidthController so it is also
functioning when in BandwidthController unit tests.

Bug: 72111305
Test: ./netd_unit_test
Change-Id: Ie60ed384b7f8d42b8e1f3417949d46895bea6572
(cherry picked from aosp commit a121e20e)