Add a trace_references_from dex optimization property

Introduce a new property `trace_references_from` in the optimize block.
This field allows specifying a set of downstream targets from which to
trace references when optimizing the current target.

In practice, this requires explicit breaking of any cycles that might
occur. A follow-up effort will explore doing this dynamically and
implicitly for `libs` references onto an optimized target that
enables this feature.

Bug: 212737576
Test: m nothing
Test: go test ./java
Flag: EXEMPT bugfix
Change-Id: I088bd8bf89403661a084a43b2822a6efbbc0294d
diff --git a/java/dexpreopt.go b/java/dexpreopt.go
index b21cfc9..e8e1cd4 100644
--- a/java/dexpreopt.go
+++ b/java/dexpreopt.go
@@ -650,6 +650,9 @@
 		// for now just exclude any known irrelevant dependencies that would lead to incorrect errors.
 		if _, ok := tag.(bootclasspathDependencyTag); ok {
 			return false
+		} else if tag == traceReferencesTag {
+			// Allow ordering inversion if the dependency is purely for tracing references.
+			return false
 		}
 		depIndex := jars.IndexOfJar(dep.Name())
 		if jarIndex < depIndex && !config.BrokenSuboptimalOrderOfSystemServerJars {