Trim leading src-dir from Ravenwood sources

Soong uses the directory structure to guess the java package, and
"annotations-src" is not a package considered for APIs. Remove the
prefix such that soong sees the correct package android.*.

Hide the Ravenwood classes for now.

Test: m checkapi
Change-Id: I430009deb7df2beac1736c4a68ff6fa1a3be572a