AMS: Add Phenotype key for proactive kills

KEY_PROACTIVE_KILLS_ENABLED controls whether proactive kills
are enabled or not.

Bug: 249601646
Test: adb shell device_config put activity_manager proactive_kills_enabled true
Test: adb shell device_config get activity_manager proactive_kills_enabled
Change-Id: I173058d77e18294a882714591dcea96dae06c12f
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>