Battery saver: Expose location power save mode as system API

- Expose the location power save mode as a system API for the fuse
provider to change the behavior.

- Didn't choose to expose PowerManager.getPoserSaveState() because the result
class seems to be a bit ugly...

- Not all the modes are exposed since the two existing saving modes are
implemented within the system server and system apps don't need to know them.

Bug: 68769804
Test: Manual test with a test app
Change-Id: I25d8e68bfb79ebbeefbc241479077389c01e0f25