[AppsFilter] make mShouldFilterCache non null

To prepare adding a snapshot cache for it. Instead of copy+modify+swap,
we now modify the content of it in place.

BUG: 218411030
Test: atest AppsFilterTest
Test: m RUN_ERROR_PRONE=true framework services.core |& grep AppsFilter
Change-Id: Ief23ef7ce2272057ebccca4f552687b21ae55c1a
1 file changed