Generate system modules for any value of EXPERIMENTAL_USE_OPENJDK9.

The flag EXPERIMENTAL_USE_OPENJDK9 controls the *default* value
of java_version / LOCAL_JAVA_LANGUAGE_VERSION, but that default
value can be overridden by individual build targets.

One requirement for individual build targets to opt-in to
java_version 1.9 is for system modules to be generated even if
the default language version is still < 1.9. We plan to allow
this soon for libcore targets; therefore, this CL changes the
system module targets to always be defined, regardless of the
value of EXPERIMENTAL_USE_OPENJDK9.

Test: In a workspace that has http://r.android.com/646840
      "make" no longer runs into the build failure from bug 76219552.
Bug: 76219552

Change-Id: I5193347452c7967f7695929619509aea0fd46844