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

(cherry picked from commit d2a975ad)

Change-Id: I9ca29e9a5a0c94234fb14a725334dae82171679b
Merged-In: I30a2a3bcfb83f502f168c0d8588d80982a5f7d76