Add property to allow activity state sharing in untrusted embedding

We choose to use a string property instead of a boolean property because
we might need to add new values for fine-grained control in the future,
and string property gives us the flexibility.

Bug: 297887697
Test: atest TaskFragmentOrganizerControllerTest
Change-Id: I29d4fe945bcbce4c9cbbde37fe7e1581cbb00f32