Back up glanceable hub enabled setting

Added the setting to backup/SecureSettings and defined the validator.

Bug: b/330945205
Test: manually with cloud backup commands
Test: atest SettingsProviderTest
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Change-Id: Ia465aca2386df1f68fad7091d43849157d7ac635