Simplify GWPC.

The allowed/blocked activities and task navigation sets are
mutually exclusive, we can keep as single set for them. This will
come in handy when adding the dynamic activity policy.

Now when the default cross navigation policy is blocking and the
blocked list is empty - then everything is allowed \o/
This is not really a bug because the policy can only be blocking
if the blocked list is not empty. But fixing for consistency.

Fixing all the nullability annotations in GWPC because the
VirtualAudioControllerTest passes nulls for a bunch of callbacks
but GWPC mostly doesn't check if they are null.

No-op, pure refactoring, so not flagging.

Bug: 298401780
Test: presubmit
Change-Id: Iadebd10fad00c5be1be28507d13bf04f1c16aec1