AMS: Add Phenotype key for low swap threshold

KEY_LOW_SWAP_THRESHOLD_PERCENT controls the minimum percentage
of free swap imposed by proactive kills.

Bug: 259307176
Bug: 249601646
Test: adb shell device_config put activity_manager low_swap_threshold_percent 0.50
Test: adb shell device_config get activity_manager low_swap_threshold_percent
Merged-In: Ieadd31efc677b43ce008a3b26726b565e9f497ab
Change-Id: Ieadd31efc677b43ce008a3b26726b565e9f497ab
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>