Adding java_sdk_library

java_sdk_library is to make a Java library that implements optional
SDK APIs to apps. It is actually a wrapper for several modules: 1) stubs
library that clients are linked against to, 2) droiddoc module that
internally generates API stubs source files, 3) the real runtime shared
library that implements the API, and 4) XML file for adding the runtime
lib to the classpath at runtime if requested via <uses-library>.

Note: this is only the initial CL for the feature. Followings are features
currently missing and under development.

1) check for API consistency
2) install stubs libs as the dist artifacts
3) ensuring that apps have appropriate <uses-library> tag
4) disallowing linking to the runtime shared lib
5) HTML generation

Bug: 77575606
Test: m -j

Change-Id: I4a4ccf6e730e041703c63bb275d8860d0de96887