Sleep softly on device close.

Add a new flag GO_TO_SLEEP_FLAG_SOFT_SLEEP. When set, this sleeps the
device only if its user activity keeping it awake. If its otherwise
staying awake because an application is holding a wakelock, then the
device continues to stay awake.

Bug: 260705307
Test: atest PowerManagerServiceTest
Test: manual
Change-Id: I08b6dacf418d927b4f86816b1b6d19965e68556a