Exempt activity from showing app handle if home is not ready/available.

There exists a timing where as we transition from SetupWizard to Home
screen, PackageManager will return null for Home availability. If Home
isn't available, we have nowhere to go for Desktop Mode, so we should
just disable it while in that state.

Bug: 359149294
Test: atest, manually
Flag: EXEMPT bugfix
Change-Id: Ia7e733d63d69266e93ee5042a23568ca5a35dc2b