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