Always use shared memory for atrace.

Remove ATRACE_SHMEM macro.

Test: atrace ss
      atrace wm
      sanity check output
Bug: 137366208
Change-Id: I1b42243678b9b9a41db18e2ff8cb3cf7bde874de
Merged-In: I1b42243678b9b9a41db18e2ff8cb3cf7bde874de