Refactored savableIds() into a SaveInfo class.

For now it's a "1-to-1" refactoring that keeps the same
functionalities, but soon SaveInfo will be expanded to
allow the AutoFillService to customize it.

Bug: 35727295
Test: CtsAutoFillServiceTestCases pass
Test: m update-api

Change-Id: I5aaa705be2b32590048f70ed0142437e05df94b7