Initial unit tests for AssistStructure

Right now this test is focused in the parcelization of a big object, due to an
upcoming refactoring on the Autofill parcelization that does not use
ensureData() to load the structure (which in turn requires calls from
system server to the app).

Bug: 35708678

Test: atest app.assist.AssistStructureTest

Change-Id: I9e1c52b19cd1d82869fa8f48113cc7dd7b00a38c