Use system modules generated by soong when using javac -target 1.9

Soong has support for building system modules.  Use the directories
produced by Soong with --system to replace -bootclasspath arguments
when using javac -target 1.9.

Since soong can't generate current SDK stubs yet, and no existing
SDK stubs need -target, only use -target 1.9 for modules that are
not compiling against the SDK.  That means in practice the only
system modules that will be used for now is the default one,
core-system-modules.

Bug: 63986449
Test: m -j EXPERIMENTAL_USE_OPENJDK9=true makes some progress
Change-Id: I350ef50aedf36fdd72458c23d4fe8a2edf1a9a02