Use bpf get first key to iterate the maps

With the newest kernel support from upstream. We can directly get the
first key of each map with a null pointer instead of using a nonexistent
key for each map. This patch replaced all related iteration process with
this new function and cleaned up all unecessary code.

Bug: 30950746
Test: ./libbpfTest ./netd_unit_test

Change-Id: Ie1fd617964d37d2a1c12b2f34457c1cc1d912571
Merged-In: Ie1fd617964d37d2a1c12b2f34457c1cc1d912571
(cherry picked from aosp commit c3bfd3ee)