Add setting/experiment for small battery devices to have

all forced app standby enabled except for when the device
is charging.

Reverting the revert of this CL. The only change is to use
EXTRA_PLUGGED instead of EXTRA_STATUS in detecting charging
state. Verified that this passes CTS test for battery saver.

Bug: 69259147
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/ForceAppStandbyTrackerTest.java
Test: atest CtsBatterySavingTestCases
Change-Id: I4f3ffc0c865a1a3a035b6d9d6838b5056b666eb9