Adding nullability annotations on ViewNode/AssistStructure.

This is in an effort to enable MissingNullability check in metalava to flag
all the new APIs that are missing nullability for APIs added since API 29.

Bug: 124515653
Test: make -j checkapi
Change-Id: I582ae992b78d738a5866c9890ef00d5b30ae7cb7
3 files changed