Stub Annotations

The annotations in these packages are used to compile the stubs. They are mostly identical to the annotations in the support library, but

(1) There are some annotations here which are not in the support library, such as @RecentlyNullable and @RecentlyNonNull. These are used only in the stubs to automatically mark code as recently annotated with null/non-null. We do not want these annotations in the source code; the recent-ness is computed at build time and injected into the stubs in place of the normal null annotations.

(2) There are some annotations in the support library that do not apply here, such as @Keep, @VisibleForTesting, etc.

(3) Only class retention annotations are interesting for the stubs; that means for example that we don't include @IntDef and @StringDef.

(4) We've tweaked the retention of some of the support library annotations; some of them were accidentally source retention, and here they are class retention.

(5) We've tweaked the nullness annotations to include TYPE_PARAMETER and TYPE_USE targets, and removed package, local variable, annotation type, etc.

(6) There are some other differences; for example, the @RequiresPermission annotation here has an extra "apis" field used for merged historical annotations to express the applicable API range.

Essentially, this is a curated list of exactly the set of annotations we allow injected into the stubs.