Fix annotation processors working w/ generated code on OpenJDK 9 toolchain

Generated code is annotated with @javax.annotation.Generated.
Annotation processors working with such code break on OpenJDK 9
toolchains because that class lives in the module
"java.xml.ws.annotation".

This CL makes that module available by adding the javac command
line flag
      -J--add-modules=java.xml.ws.annotation to javac
When the environment variable EXPERIMENTAL_USE_OPENJDK9 is set
to true.

Test: make ANDROID_COMPILE_WITH_JACK=false checkbuild tests \
      && make checkbuild tests
      (with OpenJDK 8u45 toolchain on the PATH)
Test: make EXPERIMENTAL_USE_OPENJDK9=true \
      ANDROID_COMPILE_WITH_JACK=false checkbuild
      (with jdk 9-ea+170 toolchain on the PATH)

Bug: 62050818
Change-Id: Ib3e19621a0e24f79807be0dd239e9abec3350383