Split LauncherPrefs into abs class / impl.

Splitting allows easily making a fake that doesn't have to override the implementation details of LauncherPrefs. The fake should not deal with SharedPreferences.

LauncherPrefs could be an interface, but then the companion object has more limitations. The solution there is to have a dedicated object class, e.g. `LauncherPrefItems`. I went with an abs class to make the refactor simpler.

Flag: EXEMPT refactor
Bug: 369641781
Test: go/testedequals
Change-Id: I97a2d495d3b5fa892fa53a11fb3f7a7dfb98515b