Enable Java 9 language support through java_version: "1.9" alone.

Some logic in java.go was conditional on TargetOpenJDK9(), which in
turn relies on a global build flag (EXPERIMENTAL_USE_OPENJDK9=true).
This CL changes the logic in collectBuilderFlags() to check for
javaVersion greater or equal "1.9" instead, which is true when either
EXPERIMENTAL_USE_OPENJDK9=true (global) or javaVersion "1.9" (a
per-build target attribute).

Because the value of flags.javaVersion isn't available there,
TargetOpenJDK9()-dependent logic in Module.deps() was changed to
be unconditional; as far as I understand, this means that system
modules deps will be built regardless of whether they're needed
later in the build. This will probably slow down the build
unnecessarily until the first user of these system modules appears
(expected soon), but probably not by much.

There is some TargetOpenJDK9() related logic remaining in droiddoc.go,
but this doesn't seem to break the build of Java 9 language sources
and is probably no longer needed now that metalava is used for
generating the stubs (thanks to Nan!), and those do not contain any
Java 9 language features.

Bug: 112443425
Test: Checked that Java 9 language support works on AOSP after
      this CL without a need for EXPERIMENTAL_USE_OPENJDK9=true,
      ie. after also patching CL http://r.android.com/646840 ,
      "make docs droid cts" completes, the device boots and
      Java9LanguageFeaturesTest passes.

Change-Id: I393b97a7b3bb4c1c3d06580c639e94ff6cf916ff