Disable model loading with Sandbox check instead of static boolean.

The static boolean also affects Taskbar within the Launcher process. We
only want this disabled for the Taskbar test Sandbox.

Flag: TEST_ONLY
Bug: 230027385
Test: NexusLauncherTests:com.android.launcher3.taskbar
Change-Id: Ife12e95ab34755d1aec42d79188c15232d8da092