Stop supporting System UID to embed a separate task

Not seeing a strong use case that really needs to have a
separate Task being embedded.

Bug: 211571939
Test: atest TaskFragmentOrganizerPolicyTest
Test: atest SplitActivityLifecycleTest
Test: atest TaskFragmentOrganizerTest
Change-Id: I57537a6a21b0e54762b599fd7d18b3322f6006ea