Do not proactively kill cached/empty processes post boot

Added three configs:

Do not proactively kill cached processes until user-0 is being unlocked:
(default: false)
$ device_config put activity_manager no_kill_cached_processes_until_boot_completed true

Do not proactively kill cached processes for 10 minutes after any user is being unlocked:
(default: 0)
$ device_config put activity_manager no_kill_cached_processes_post_boot_completed_duration_millis 60000

Also added the following config to control MAX_EMPTY_TIME:
(default: 30 minutes)
$ device_config put activity_manager max_empty_time_millis 1800000

Bug: 222365734
Test: Manual test

Change-Id: I48c0d6dd5f1836159be458ff00e58a59d9987dcf