Update battery saver preference key

The updated preference key made slice auto conversion mechanism fail,
so when this slice is requested by external apps, the UI won't be shown
properly. So we need to revert the key to the original one.

Bug: 131897855
Test: manual test
1. Open Settings app -> Settings search -> search battery saver
2. Observe battery saver result item has a toggle

Change-Id: Ifb5b0a6786d60d1e67567272610cc3cb078f11a4
Merged-In: Ifb5b0a6786d60d1e67567272610cc3cb078f11a4