Add D8 support

Bug: 67754178
Test: m -j32 checkbuild && USE_D8=true m -j32 checkbuild

Change-Id: If63afc10ceb5e753bbb7f195bb8a895eaef10775

java/config/makevars.go

@@ -43,6 +43,8 @@ func makeVarsProvider(ctx android.MakeVarsContext) {
 	ctx.Strict("JAR_ARGS", "${JarArgsCmd}")
 	ctx.Strict("JAVADOC", "${JavadocCmd}")
 	ctx.Strict("COMMON_JDK_FLAGS", "${CommonJdkFlags}")
+	ctx.Strict("DX", "${DxCmd}")
+	ctx.Strict("DX_COMMAND", "${DxCmd} -JXms16M -JXmx2048M")
 
 	if ctx.Config().IsEnvTrue("RUN_ERROR_PRONE") {
 		ctx.Strict("TARGET_JAVAC", "${ErrorProneCmd}")