Sticky battery saver

- When battery saver is enabled manually (i.e. via PM.setPowerSaveMode()),
it'll stick, and we'll re-enable battery saver even after a reboot
or a charge.

- Extracted all battery saver state transition logic into a separate
class.

Fix: 75033216
Bug: 74120126
Test: Manual test with "dumpsys battery set ...."
Test: atest $ANDROID_BUILD_TOP/frameworks/base/services/tests/servicestests/src/com/android/server/power/batterysaver/BatterySaverStateMachineTest.java
Change-Id: If020cd48f341b339783fe09dd35bc7199e737a52
Test: dumpsys power
Test: incident_report power
Test: atest CtsBatterySavingTestCases
diff --git a/core/tests/coretests/src/android/provider/SettingsBackupTest.java b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
index a504ab9..6ef773a 100644
--- a/core/tests/coretests/src/android/provider/SettingsBackupTest.java
+++ b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
@@ -266,6 +266,7 @@
                     Settings.Global.LOW_BATTERY_SOUND_TIMEOUT,
                     Settings.Global.LOW_POWER_MODE,
                     Settings.Global.LOW_POWER_MODE_TRIGGER_LEVEL_MAX,
+                    Settings.Global.LOW_POWER_MODE_STICKY,
                     Settings.Global.LTE_SERVICE_FORCED,
                     Settings.Global.MAX_NOTIFICATION_ENQUEUE_RATE,
                     Settings.Global.MAX_SOUND_TRIGGER_DETECTION_SERVICE_OPS_PER_DAY,