Moved string-formatting methods from Slog to Slogf.

Slog is a core class provided by frameworks.jar, and these new methods
were causing performance regressions, so they were moved to a
system_server only class.

Bug: 182476140
Bug: 183523451

Test: atest --rebuild-module-info FrameworksMockingServicesTests:SLogfTest

Change-Id: I98cbb122d5410b61812edc85162aa6b10b995fdc