Allow ADPF WorkDuration CPU duration to be zero

This patch updates the checks, tests, and documentation to ensure ADPF
CPU == 0 is allowed, and instead checks to make sure that both CPU
and GPU cannot be 0 at the same time.

Bug: 323226967
Test: atest PerformanceHintManagerTest
Test: atest WorkDurationTest
Test: atest HintManagerServiceTest
Change-Id: I6aea1d6e5d445ba21af72ac8187cc1ce15ce8b05