Define new config for Private space GAIA screen

Allow partners to disable GAIA education screen during private space account login.
By default this is set to true which will be used in Pixel.

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

Bug: 344601737
Test: Build

Change-Id: I82dcf29105be434b406274d9029dbbdf28c7433f