One fewer DependencyProvider

`grep -r "(class|interface) DependencyProvider" returns 3 separate
entities claming to be DependencyProvider, which is a little confusing
and unnecessary. Privatized and renamed Dependency.DependencyProvider to
Dependency.LazyDependencyCreator, since I think that's a little bit
closer to what it is

Test: build systemui; atest SystemUITests
Change-Id: I2eed5be889b6c75324c2cf5cfef5d55827dbd8c5