Replace generic GC type flag with a specific Generational CC flag.

The "gctype" device configuration flag (from the "runtime_native_boot"
namespace) was a string passed verbatim as an argument to the runtime
option "-Xgc". It was too generic, conveyed no typing information, and
was error-prone (there was no control over what was passed from the
server to the zygote, and then to the runtime).

This change replaces "gctype" with a specific, Boolean
"enable_generational_cc" flag. This new flag better reflects the
nature of the experiment to be conducted (either enable or disable
generational garbage collection in ART's concurrent copying
collector).

Test: core/jni/runtime_native_boot-flags-test.sh
Bug: 72446017
Bug: 120794191
Bug: 123754583
Change-Id: I30a73171c0dc3c7bc891c4f164eed0ba42b0f420