Move UsageGraph from SettingsLib to Settings.

In preparation for modifying the graph code to show detailed projections
(see bug), this change moves it into Settings along with related tests
and resources.

Bug: 38400320
Test: manual, make SettingsUnitTests, make SettingsGoogleUnitTests
Change-Id: I88e5336c15827727b3427e29b10954bba9cfba7d