Fix proguard_usage.zip with USE_RBE=true RBE_R8=true

proguard_usage.zip contained an empty unused.txt file when
the unused.txt file wasn't specified as an output of the r8
rule.

Test: m USE_RBE=true RBE_R8=true out/soong/.intermediates/frameworks/base/packages/Shell/Shell/android_common/proguard_usage.zip
Change-Id: Ie6a7c8b2581fe21d643e675143d6cc0e2ce80456

java/dex.go

@@ -135,6 +135,7 @@ var r8, r8RE = remoteexec.MultiCommandStaticRules(pctx, "r8",
 		"$r8Template": &remoteexec.REParams{
 			Labels:          map[string]string{"type": "compile", "compiler": "r8"},
 			Inputs:          []string{"$implicits", "${config.R8Jar}"},
+			OutputFiles:     []string{"${outUsage}"},
 			ExecStrategy:    "${config.RER8ExecStrategy}",
 			ToolchainInputs: []string{"${config.JavaCmd}"},
 			Platform:        map[string]string{remoteexec.PoolKey: "${config.REJavaPool}"},