[Reland] 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.

Original change did not work because the doReturn() was defined prior to adding WPC as a mock (my bad when rebasing).

Test: atest com.android.server.am.MockingOomAdjusterTests
BUG: 370890207
Flag: com.android.server.am.perceptible_tasks
Change-Id: I7855fb86c5897164fcd14bf21b7cce87f32a3d18