Fix build error due to null annotations in core platform API

Change https://r.android.com/1667508 adds nullability annotations to
the core platform APIs which broke this code with the error:

com.sun.tools.javac.code.Symbol$CompletionFailure: class file for androidx.annotation.RecentlyNonNull not found

This change adds the stub-annotations library to provide the definition
for that class.

Bug: 183938110
Test: m checkbuild
Change-Id: I8f9560aaa314a47d85b8d4c33c8fe285b2eba194
1 file changed