Let @NonNull be the default.

Use package-info.java with a new `@NonNullByDefault` annotation
to specify that `@NonNull` is the default, and stop relying on
sprinkling our own `@NonNull` annotations everywhere instead.

This follows the generic Google recommendation and follows the approach
taken by some teams:
https://engdoc.corp.google.com/eng/doc/devguide/java/practices/null.md#legacy
go/android-media-solutions-coding-conventions#use-nullable

We still rely on `@NonNull` for all of our API surface though, as per
go/android-api-guidelines#annotations-nullability

Bug: 260989630
Test: builds
Change-Id: I99ab8f3d94875c8fceca7c9f968b7d8d652d0999