Regenerate API signatures to add missing nullability annotations

A bug in metalava prevented nullability annotations being merged onto
public methods that were declared on a non-public class and then
copied down onto its public super classes. This regenerates the API
signature after fixing the bug.

Fixing the metalava bug also caused it to detect invalid nullability
annotations on the StringBuilder class due to missing annotations which
have also been fixed as part of this.

Bug: 143700871
Test: m update-api && m checkbuild
Change-Id: If9e44e9cece2291b152f3d089c14dc0e41e9e037