Add R8 support

Add support for R8 to optimize apps and java libraries.

Test: m checkbuild
Change-Id: I2afd5d7a84912d3ab613c32c599bd1ebe60562e0
diff --git a/java/config/config.go b/java/config/config.go
index 930e65e..75176c9 100644
--- a/java/config/config.go
+++ b/java/config/config.go
@@ -106,8 +106,8 @@
 			return path.String(), nil
 		}
 	})
-
 	pctx.HostBinToolVariable("D8Cmd", "d8")
+	pctx.HostBinToolVariable("R8Cmd", "r8-compat-proguard")
 
 	pctx.VariableFunc("TurbineJar", func(config android.Config) (string, error) {
 		turbine := "turbine.jar"