Stop restoring spell checker settings

This CL logically reverts the following CL (and some subsequent
changes) to stop restoring spell checker related secure settings.

 * Ib8382f0296f0726b64494d3b1fd8237e13adb540
   06cbaddb

Reason for revert:
Although we believe it would be great if we can seamlessly migrate to
a new phone with keeping spell checker related settings, there still
remain several tricky scenarios.

 * We are not ready to distinguish whether a certain spell checker
   related setting was explicitly set by the user or programmatically
   set by some components in the previoud device.  This includes the
   case where TextServicesManagerService (TSMS) itself automatically
   updates those settings e.g. by selecting a default spell checker
   service from the pre-installed ones.  We are not sure if trying to
   migrate such an auto-selected setting to a new device actually
   makes sense, especially if it happens without any user
   confirmation.

 * We have a strict rule about what spell checker service can be
   selected automatically, and the rule has been that only
   pre-installed spell checker services can be automatically selected
   by the system, unless some system components that have
   WRITE_SECURE_SETTINGS permission overrides it. Mechanically
   selecting a spell checker service just because it was enabled in
   the previous device may not fit this model well.

 * Unlike IMEs, currently the Android OS allows only one spell checker
   service to be enabled.  This means that if the new device doesn't
   have the corresponding spell checker service, the user will lose
   spell checking functionality even if the device pre-installs
   functional spell checker service.  This problem is hard to notice
   because unlike IMEs spell checker service does not have its own UI.

 * Also unlike IMEs, spell checker related secure settings are still
   hidden and not published as public APIs.  Those settings do not
   have no official compatibility story across devices yet.

 * It is also possible that the default spell checker service in the
   previous device is not published to all the devices thus there is
   no way for the the new device to install it.

This CL therefore excludes spell checker related settings from
backup/restore, as a short-term answer to above scenarios until we
come up with better ideas to deal nicely with them.

Bug: 110367605
Test: atest FrameworksCoreTests:android.provider.SettingsBackupTest
Test: atest FrameworksCoreTests:android.provider.SettingsValidatorsTest
Change-Id: I8e4a0d4b3b758a84d5a075fa52851b1e8dd707eb