use mapRetrieveWO() and retrieveProgram() instead of bpfFdGet()

straightforward replacement to use new funcs from BpfUtils.h

Test: atest libtimeinstate_test - proves it builds, and everything fails like before
  'git grep bpf_obj_get_wronly' comes up empty
Bug: 150040815
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Id2459eca2a66c257e36ffcfb628150da44f90c81