Dream settings refactor.

We will be re-using the grid UI for the dream complication picker, so
this change refactors some of that logic so it can be re-used in the
complication controller.

We also update some of the UI here to match the most recent UX
decisions.

http://screen/BxhAwZwQSVoxUmN.png

Test: locally on device
Test: make -j64 RunSettingsRoboTests ROBOTEST_FILTER="com.android.settings.dream.DreamPickerControllerTest"
Bug: 214250590
Bug: 215703483
Change-Id: I9fbfa0f0cd31ae3d119c7cd1a9562d4cdb203d6c