Consolidate the collectReachable algorithm to one implementation

Any discrepencies in what different alogrithms consider reachable could
cause difficult to identify bugs. Consolidating to one implementation
guarentees the concept of "reachablity" remain consistent.

Flag: com.android.server.am.consolidate_collect_reachable
Fixes: 364936739
Test: atest MockingOomAdjusterTests
Test: atest BroadcastQueueTest
Change-Id: If64bc46b4814a2dfa018ab6c4b35f94d80d683da