Read "gctype" flag from namespace "runtime_native_boot".

ART does not support changing the garbage collector type/configuration
dynamically at the moment, so we can only change it when the runtime
starts, i.e. at boot time.

Also materialize "gctype" and "runtime_native_boot" as named constants.

Bug: 120794191
Bug: 72446017
Test: adb shell device_config put runtime_native_boot gctype nogenerational_cc && adb reboot
Test: adb shell device_config put runtime_native_boot gctype generational_cc && adb reboot
Change-Id: Ibf765a5f442136d6327be99786ea9ae4b4537d59