Add new Always on display slice

Currently we create Always on Display slice through
AmbientDisplayAlwaysOnPreferenceController. However, as the Always on
Display is merged into Idle lock screen, the conversion won't work
properly anymore when devices support the aware sensor.

Hence, we create another custom slice for this purpose.

Bug: 145920511
Test: manual & robotest

Change-Id: Ifd90e2de5219bd4e97aa13b5855fdab95ff6c4d0