LS Wallpaper Sampling for LS Smartspace

Refactoring RegionSampler implementations (LS smartspace) to
properly sample from lockscreen wallpaper since it is no longer an image
view.

This also addresses an edge case where Wallpaper Colors was incorrectly
calculating the hint for applying dark text on an all-white
wallpaper/region if it was very small.

This change also introduces new unit tests for Region Sampler to ensure
there is full test coverage for this feature.

Flag: region_sampling
Bug: 265968912
Test: manual, atest WallpaperColorsTest, atest RegionSamplerTest
Change-Id: Ic46c7c86bf8bc28eb0cb01c202f785a181c83564