Allow restore of settings with nullable components

Creates a new settings validator that allows for null components, which
have semantic meaning for certain settings (in this case, that no
service is selected).

Bug: 79925290
Test: 1) atest SettingsValidatorsTest
2) Manual:
- In Settings UI, select "None" for autofill service and accessibility
shortcut target
- "adb backup -keyvalue -f nullsettings.ab com.android.providers.settings"
- "adb restore nullsettings.ab"
- Verify no crashes and that autofill service and accessibility shortcut
target both have "None" in Settings UI

Change-Id: I159b3f4706c6b981a30437c31724b106eb4e3f2a