Dismiss Inattentive Sleep Warning when inattentive sleep is disabled

Make sure that if attentive_timeout is set < 0,
maybeHideInattentiveSleepWarningLocked is still called to dismiss the
warning.

Bug: 191822171
Test: Show warning:
        adb shell settings put secure attentive_timeout 30000
      Disable inattentive sleep:
        adb shell settings put secure attentive_timeout -1
      Verify warning is dismissed
Test: atest PowerManagerServiceTest#testInattentiveSleep_hideWarningIfInattentiveSleepIsDisabled
Test: atest --rerun-until-failure 20 InattentiveSleepTests#testInattentiveSleep_noWarningShownIfInattentiveSleepDisabled
Change-Id: I9e5f956924bba9f29ae6cac94b087bd73ed0bac0