Add a flag for screen timeout policy listener APIs

Protects changes added for the screen timeout policy
listener API behind an aconfig flag.

Test: atest PowerManagerServiceTest
Bug: 363174979
Flag: com.android.server.power.feature.flags.enable_screen_timeout_policy_listener_api
Change-Id: I8c722b223931613ac54e7c3f50c57f72dc059f7b