Add system_modules to droidstubs

This allows droidstubs to use the same system modules to create the
stubs that will be used to compile them. It improves consistency and
avoids droidstubs having to duplicate the libraries that make up the
system modules on its libs property.

Adds systemModules() to the sdkContext which allows consistent error
checking behavior between droidstubs and java_library.

Bug: 142534789
Test: m checkbuild
Change-Id: Ib2006906d9528a900f16851f50b62152ffb51a1b