Merged and dist lists of unused methods from R8

Merge all the proguard_usage.zip files produced by the R8 rules and
dist the result.

Bug: 151857441
Test: m TARGET_BUILD_APPS=DocumentsUI dist
Change-Id: I7e6d73241478016093a203dc7bd86407ab86a4ac
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 307c2c2..79f1bb4 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -275,6 +275,7 @@
 LOCAL_SOONG_LINK_TYPE :=
 LOCAL_SOONG_LINT_REPORTS :=
 LOCAL_SOONG_PROGUARD_DICT :=
+LOCAL_SOONG_PROGUARD_USAGE_ZIP :=
 LOCAL_SOONG_RESOURCE_EXPORT_PACKAGE :=
 LOCAL_SOONG_DEVICE_RRO_DIRS :=
 LOCAL_SOONG_PRODUCT_RRO_DIRS :=