Guard regionSampler registering/deregistering code behind the flag.

When the flag is off, regionSampler was being created and registered, but
it actually does nothing. So guard this code behind the flag to avoid
confusion.

Note: I noticed this while debugging b/254095511. But this current CL
isn't required for the fix for the bug (ag/20597942).

Bug: 202758428
Bug: 254095511
Test: no-op. Passes existing tests.

Change-Id: I391b788debbb0e3063e72745c09b1e94b027ac83