framework: fix setDisplayContentSamplingEnabled rc

Return code was documented as true-as-success, but test
was implemented as false-for-success. Correct code so the
doc is correct.

Test: atest FrameworksServicesTests:DisplayManagerServiceTest
Change-Id: Ifc1ecda24735b64942090527866d19d6f75cda8b
Fixes: 122483211