Add exported_plugins to java.Library.

The behaviour is similar to go/be#java_library.exported_plugins. Plugins added to exported_plugins of library X are not applied to the library itself, but rather to libraries that directly depend on the library X.

Test: m checkbuild
Bug: 139740873
Change-Id: I4042bd482ad9cb12d6fbaac51f039d38b1b7a428