java_sdk_library: Allow creation of impl shared library to be disabled

Most modules will be providing their implementations via APEX and so do
not need to create an implementation shared library as part of this.

Adds an api_only property which will:
* Prevent the creation of the implementation library.
* Prevent the creation of the .xml file needed at runtime to make
  the shared library available.
* Prevent the library being added to the list of java sdk libraries
  used by make to handle installation.

Bug: 145998881
Test: m checkbuild
Change-Id: Ida5e46a81aa5b0a041882d90d5f362ec79fdddb2