refactor bpf maps into class

Use a class object to wrap up the bpf map file descriptor and provide
some base function to look up, write and delete the map content. The map
class also have a nested iterator class to iterate over the map and two
helper function to simplify the iterating process. Removed the
mDeleteStatsMutex since it cannot prevent framework side to read the
stats while we are deleting the stats and all the other netd operation
on the stats related map doesn't need to iterate through the map anyway.

Bug: 78250686
Test: netd_unit_test libbpf_test
Change-Id: I358ba65f7022fd03f8ca573550055734052b6fd2