Use java language 1.9 for sdk_version: "current"

Java language was set for 1.8 for anything building against the
current SDK because the stubs were not built in Soong, so the
system modules could not be built.  The stubs have been built
in Soong since Iabd32b30954b3f4a6d9a779fde52a032b684807e, but
I5e870c34dd0ebc8ae3f888ec627da590c846a76f missed updating
this TODO.

Use 1.9 when building against the stubs, but continue using
1.8 for unbundled builds until we have prebuilt system modules.

Always use the core-current-stubs-system-modules to avoid
splitting android.* packages between the system modules and
the classpath, which would cause new classes in android.*
packages in classpath jars to be ignored.  Add a new
java9Classpath field to sdkDep that will contain the stubs jar
when targeting Java language level 1.9, and plumb it through
to javac and turbine.  Rename the modules field to bootclasspath.

Bug: 142896162
Test: m checkbuild
Change-Id: Icfd32d0a863b2303a997c7cf03cb3708aade4724