Allow home activity to be resizeable if it explicitly wants to be.

We previously didn't allow home activities to be resizeable due to not fully
understanding how the UX would work. We now have a better idea of how the UX will
work. However, since this is a behavior change for home activities we require them
to explicitly say they want to be resizeable vs. the framework assuming they can
be resizeable based on their target sdk.

Bug: 30982291
Test: cts/hostsidetests/services/activityandwindowmanager/util/run-test android.server.cts
Change-Id: I41aefc8c3b3da4c2cf66be81835222196df3ef26