Add gctype device config property for GC experiments

Added to the runtime_native namespace.

The new property overrides dalvik.vm.gctype if it's set.

Bug: 120794191
Test: adb shell device_config put runtime_native gctype CC,preverify
Test: adb reboot

Change-Id: I30a2a3bcfb83f502f168c0d8588d80982a5f7d76