Define new config to enable Private space Hide Illustration

Allow partners to disable the search tile content and hide illustration
in Hide Private Space Settings page.
By default this is set to true which will be used in Pixel.

OEM can set value of config_enableSearchTileHideIllustrationInPrivateSpace in the device
overlay file to false to disable it.

Bug: 346612477
Test: Build
Flag: EXEMPT resource only update
Change-Id: I3d9a932edbcf91dc5a94177d48a3d798dbdb2405