Remove sleep_timeout setting from backup

It is not user-configurable.

Bug: 79779285
Test: atest SettingsBackupTest
Test: # w/ LocalTransport
      adb shell settings put secure sleep_timeout 5000
      adb shell bmgr backupnow com.android.providers.settings
      adb shell settings put secure sleep_timeout 3000
      adb shell bmgr restore 1 com.android.providers.settings
      adb shell settings get secure sleep_timeout => ensure 3000

Change-Id: I68bf2f91cc3d4c00463468a6abaef3b64d5c10b9