Backup spell checker settings (again).

We stopped backing up spell checker settings in
  commit 1893bd16
The main concern was that enabling a (non-system) Spell checker from
a previous device may not work on the new device, which may have
a different default Spell checker or different security policies.

This CL does the following two things to address the forementioned
concern:
(1) Backup and restore spell checker on/off state, regardless of what
    spell checker is used. This part should not be problematic,
    as long as the new device supports Spell Checker functionality.
(2) Backup selected spell checker and its language, and restore them
    only when the back-ed up spell checker is a system app in the
    destination device.

Bug: 314702821
Bug: 314703108
Test: atest SettingsProviderTest
Flag: EXEMPT bugfix
Change-Id: I98225d0b8dc51c734622ba3b60da41d799338261