Fix failed MockingOomAdjusterTests when Perceptible Tasks is Enabled

The `WindowProcessController.getPerceptibleTaskStoppedTimeMillis()` is mocked and returns 0 by default which is wrong. Need to update to behave similarly to the real method, where tasks that are not enabled for perceptible tasks should return Long.MIN_VALUE by default.

Test: atest com.android.server.am.MockingOomAdjusterTests
BUG: 370890207
Flag: com.android.server.am.perceptible_tasks
Change-Id: I12ac5c022b5ac261416346c9537548fcd26e4b2c