Make 'make docs' work on OpenJDK 9 toolchain.

OpenJDK 9's javadoc tool doesn't support the -bootclasspath
command line option, even with -source 1.8.

Instead, under OpenJDK 9, javadoc needs to be passed a
--patch-module argument to tell it the location of the
classes patching packages from java.* modules.

The source files in libcore/{dalvik,libart,luni,ojluni} and
external/icu/android_icu4j that go into PRIVATE_BOOTCLASSPATH
patch packages from the modules java.base, java.desktop,
java.logging, java.prefs, java.sql, jdk.net, and
jdk.unsupported. However, this CL takes the simpler approach
of placing them all in java.base, which appears to work for
the purposes of the javadoc run.

Test: Ran the following both on OpenJDK 8 toolchain and on
      OpenJDK 9 (with EXPERIMENTAL_USE_OPENJDK9=true):
      rm -rf out/target/common/docs/ && make docs
Test: Compared (via meld) the contents of out/target/common/docs
      for the two toolchains (there were some differences, see bug).
Bug: 62049770

Change-Id: If3dd927477ca32dab7ffb528350de872e1017184