BandwidthController: fix performance-no-automatic-move warning.

In the next compiler update, it reports the below warning as error:

system/netd/server/BandwidthController.cpp:263:12: error:
constness of 'ipt_basic_accounting_commands' prevents automatic move
[performance-no-automatic-move,-warnings-as-errors]
    return ipt_basic_accounting_commands;

Bug: 149839606
Test: build.
Change-Id: I32ee491b0de6a986dc66de06a4f56d980f794136
Merged-In: I32ee491b0de6a986dc66de06a4f56d980f794136