Bug fix for device idle mode light

Before, 'adb shell dumpsys deviceidle force-idle light' would
always fail because it would call stepIdleStateLocked, which
does not affect mLightState. It now calls stepLightIdleStateLocked,
which modifies mLightState andputs the device in doze mode light.

Test: ran 'adb shell dumpsys deviceidle force-idle light' and
verified device entered doze mode light.

Change-Id: If8e6a1a0355c71c01727cdc64fa8e2ac82203e98