javac 9 -target 1.9: Move bootclasspath jars onto classpath.

When building with OpenJDK 9's javac with -source 1.9 -target 1.9,
a runtime image (with system modules) is used via the --system
command line flag, instead of --bootclasspath.
The runtime image only contains the default libraries (libcore)
but is missing other libraries that might be on the bootclasspath
for a particular build target.

This CL fixes compilation for this case by adding the missing
jars onto the classpath instead.

This already used to work but was broken by CL [1]. I attempted
to conditionally revert some of that CL's changes to java_common.mk
for the case of language level 1.9, but couldn't get it to work.
Therefore this CL follows a different approach.

[1] http://r.android.com/519552

Bug: 38177569
Test: Treehugger
Test: Ran "EXPERIMENTAL_USE_OPENJDK9=true make checkbuild docs"
      and spot-checked some javac invocations to confirm that the
      system modules libs were not included in the bootclasspath,
      but other libs were.

Change-Id: I48fd11aac9b310bfa58dee0f9cfb3ef33f10bca8