Add a constant for low perceptible adj

NotificationListeners were moved to below FGS so
that they could be killed to relieve memory pressure.
Adding a proper place for it at oom adj 250.

Bug: 79848281
Test: Verify oom_adj value using dumpsys activity

Change-Id: I6059243b50921a696e967d88a7b21cbc6107856b
4 files changed