Set Multi-Gen LRU enabled based on experiment prop

Experiments can enable/disable MG-LRU using the
persist.device_config.mglru_native.lru_gen_enabled property
which will update the coresponding sysfs control to enable or
disable this feature in the kernel.

Test: adb shell device_config put mglru_native lru_gen_config [none, core, ...]
Test: verify MG RLU is enabled/disabled: cat /sys/kernel/mm/lru_gen/enabled
Bug: 227651406
Bug: 228525049
Change-Id: I0708df8c78a85359d5cb6d5b167836768029380e