Revert "Revert "[AppsFilter] read-only interface for snapshots""

This reverts commit 2e06c600.

Reason for revert: fixing the boot time regression

Provides a read-only interface that is used by computer and snapshots.
It fixes the data conflicts when AppsFilter is changed while a snapshot is
taken.

Test: atest AppsFilterTest
Test: atest com.android.server.utils.WatcherTest
Test: m RUN_ERROR_PRONE=true framework services.core |& grep AppsFilter
BUG: 218411030
Change-Id: I06624ea79ff85160488a6c7863213b0c44fe2154