Add a soong wrapper for R8's tracereferences

Add a new traceReferences rule that wraps the base R8 tracereferences
command. This allows generating a set of keep rules after tracing
references from a given set of sources onto a given target jar.

This will be used to keep hidden/unannotated members of optimzed
libraries that may be referenced by other bundled libraries on the
platform.

Bug: 212737576
Test: m
Flag: EXEMPT basic soong rule addition
Change-Id: I92cbe8839ce68056311c7c38de181f2e5f7cb775
diff --git a/java/Android.bp b/java/Android.bp
index 911af83..99d9c38 100644
--- a/java/Android.bp
+++ b/java/Android.bp
@@ -78,6 +78,7 @@
         "system_modules.go",
         "systemserver_classpath_fragment.go",
         "testing.go",
+        "tracereferences.go",
         "tradefed.go",
     ],
     testSrcs: [