Implement perceptible tasks

Add a hidden API to allow specifying that a task is perceptible. All
activities in the task will have an oom_adj score of initially
perceptible, and they will be treated as active for the purpose of
computing ActivityManager#isUidActive(). oom_adj is downgraded to
previous_app after a timeout period (currently 5min)

Bug: 370890207
Test: atest
com.android.server.am.MockingOomAdjusterTests#testPerceptibleAdjustment
+ Manual testing
Flag: com.android.server.am.perceptible_tasks

Change-Id: I3cbc2c4d0fb8cac9d3bce906aaa32579d41b2987