Modernize string handling in BandwidthController

This change is preparation for removal of xt_quota2 in favor of NFLOG.
Note that the scope of changes is mostly limited to mechanical single
line changes from "const char*" to "const std::string&".

Test: as follows
    - built
    - flashed
    - booted
    - "runtest -x .../netd_unit_test.cpp" passes
    - "runtest -x .../netd_integration_test.cpp" passes
Bug: 38143143
Bug: 28362720

Change-Id: I56ba810ff6fa2f409e32d86508cfdb1a81a50a4e