Fix flaking DeviceIdleController tests.

The mock DeviceIdleController.Constants object had all of its values set
to 0, so setAlarmSoon wasn't working properly because it would set the
alarm to now + 0, which would have passed by the time the next line in
the test came around.

Bug: 118639768
Test: atest com.android.server.DeviceIdleControllerTest
Change-Id: Ie485e9319d478e4887dc49e4715ddd6ccce9e768