Add @Nullable and @NonNull to module API

These two annotations are already in the public/system stubs,
but are re-written to be package private instead of public. This
makes them inaccessible to libs building against these stubs.

When setting sdk_version:<X>, the stubs of X will appear before any
other libraries on the classpath. This also makes it impossible for
modules to add Nullable annotations of their own when setting sdk_version.

Exposing these annotations as public in the module stubs makes it
possible for modules using these annotation to specify an sdk_version.

Bug: 146758669
Test: m
Test: downstream CLs using sdk_version:module_current
Change-Id: I71fb766d02f833ed9a1b3a7dbfbc1c8118882b30