Force enable DeviceIdleController in imperceptible killing test

Some devices (i.e. cf) have configured the DeviceIdleController off
by default.

Bug: 151357358
Test: atest CtsAppTestCases:\
      ActivityManagerTest#testKillingPidsOnImperceptible
Change-Id: I33eece7fcc022ee68ab97daffa6adb77d8951742
1 file changed