Skip backup/restore of wallpaper package

Do not backup or restore wallpaper package when
'backup_skip_user_facing_data' is set. See https://b.corp.google.com/issues/153940088#comment2 for context.

Bug: 153940088
Test: 1. atest UserBackupManagerServiceTest
      2. atest PerformUnifiedRestoreTaskTest
      3. atest CtsBackupTestCases
      4. atest CtsBackupHostTestCases
      5. Manual:
          1) adb shell settings secure put backup_skip_user_facing_data
	  1
	  2) adb shell bmgr backupnow com.android.wallpaperbackup ->
	  verify skipped
	  3) adb shell bmgr restore 1 com.android.wallpaperbackup ->
	  verify skipped
	  4) adb shell settings secure put backup_skip_user_facing_data
	  0 -> verify com.android.wallpaperbackup is backed up /
	  restored

Change-Id: Iedd0c4030a635e294a04d35bd525ca852c929b92