Only report uncached nulls in PIC

dumpsys cacheinfo now only reports nulls that are not cached.  This
makes it easier to identify caches that do not cache nulls and which
might benefit from caching nulls.

Flag: EXEMPT bug-fix
Bug: 378762206
Test: atest
 * FrameworksCoreTests:PropertyInvalidatedCacheTests
 * FrameworksCoreTests:IpcDataCacheTest
 * CtsOsTestCases:IpcDataCacheTest
Change-Id: If01b5c73492c9fd626163c9f2ac923f8b7fbab79