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
Change-Id: I6927899793c7d9801541465280af32627e200dc9