Introduce oomadjuster_cached_app_tiers flag

This change adds a flag for cached app tiers in OomAdjuster. The
default-disabled constant for this functionality has been replaced with
a flag check.

Test: Build successfully, check that the flag exists.
Bug: 369893532
Flag: com.android.server.am.oomadjuster_cached_app_tiers
Change-Id: I71445a19328724f77ad489d45433ce82be4be529