Revert heap growth multiplier logic for Generational CC collection.

Have Generational and Non-Generational CC collections both use
Heap::HeapGrowthMultiplier directly in Heap::GrowForUtilization.

Test: ART run-tests & gtests, libcore tests, JDWP tests (host & device)
Test: Device/emulator boot test
Bug: 67628039
Bug: 12687968
Change-Id: Ie4b8aa9dc190ed8a9d069b2dc957c50e7a0ef05a