Add missing nullability annotations.

To prepare for enabling MissingNullability Metalava check this CL
works on adding missing nullability issues that metalava flags if
we tell it to flag new things since API 29.

This is not a complete CL, mostly addresses public api and
toString/equals for @SystemApi

Exempt-From-Owner-Approval: Large scale nullability clean up
Bug: 124515653
Test: make -j checkapi
Change-Id: I109260842cfc25f06e40694997fcbb4afa02c867