Remove sdkPreSingleton and overlaySingleton

These were the only 2 pre-singletons in soong. sdkPreSingleton is
totally unused. overlaySingleton can be done during
GenerateAndroidBuildActions instead.

Test: m nothing --no-skip-soong-tests
Change-Id: Ieb5ab92f18cb56be4049c0842f61df8aa02dc52c