Test for getFirstMapKey of bpf maps

The upstream commit (8fe45924387be6b5c1be59a7eb330790c61d5d10 bpf:
map_get_next_key to return first key on NULL) introduce the support for
bpf maps to return the first key of the map when receive a NULL key from
userspace. This change will test this functionality in supported kernels
to make sure they have the correct backport.

Test: ./bpf_test.py
Bug: 30950746
Change-Id: I6a497a3ccb518ebb037bfa447f827d2715271291
Merged-In: I6a497a3ccb518ebb037bfa447f827d2715271291
(cherry picked from aosp commit 493cfb50)