Add a private API to set mocking hint manager for testing

Updated the doc of interval methods as they won't fail now

Bug: 346604998
Flag: EXEMPT ndk
Test: atest NativeSystemHealthUnitTest SystemHealthManagerUnitTest
            NativeSystemHealthTest HeadroomTest
Change-Id: Ia8a3d921f831ee0c07b16926500581fc2a8d5b00