Use injected main thread handler in LightBarTransitionsController

The Handler within `LightBarTransitionsController` was being instantiated without an explicit Looper, causing it to default to the Looper of its creation thread. In scenarios where its parent, `SystemUIDisplaySubcomponent` (after ag/35378953), is created eagerly on a background thread, this resulted in the Handler also operating on a background thread, which is not suitable for its intended UI operations

Flag: EXEMPT a small bug fix
Bug: 441940469
Test: atest SystemUITests:LightBarTransitionsControllerTest
Change-Id: I37ed48b991e61064ffe73e5d631fe7aadd160a52