Prevent cropHint been overwrite by display size.

Follow the issue discussion, the cropHint should be preserved
as what is backuped, so it shouldn't be overwrite by any reason.
Validate the cropHint is enough.

Bug: 152293002
Test: atest WallpaperManagerServiceTests
Test: run bmgr to do backup/restore and see the cropHint doesn't
been overwrite after reboot.

Change-Id: I7b35ef17601418d730c3bcfc4c7e24978e55c721