Add fallback holder for home role behavior.

There is logic inside PackageManagerService and other parts of the
system that allows using a home app without setting it as the default
home (i.e. being configured as the preferred activity for home
intent). This won't work in the new role mechanism because if it's
used as the default home app, it should be set as the default home
app, and setting an app as default home app means configuring it as
the preferred activity.

The only behavior change is that when a user installed another home
app, and they were using the only home app and it wasn't manually
selected so not configured as the preferred activity, they will not be
presented with an intent disambiguation dialog when pressing home. But
that's not a very big behavior change, because when they install a
third home app, they will have to go to settings to change it anyway.

Bug: 124260975
Test: manual
Change-Id: Ide32c5fc218ba6e19dd45593d58a61c81fe03e9e