Allow missing classes in R8 builds

Bug: 180878971
Bug: 72683872
Test: TARGET_PRODUCT=aosp_arm64 m -j core-oj
Change-Id: I663220567eb8f034d3efccc60228aef91c716e8e

java/dex.go

@@ -260,6 +260,9 @@ func (d *dexer) r8Flags(ctx android.ModuleContext, flags javaBuilderFlags) (r8Fl
 		r8Flags = append(r8Flags, "--debug")
 	}
 
+	// TODO(b/180878971): missing classes should be added to the relevant builds.
+	r8Flags = append(r8Flags, "-ignorewarnings")
+
 	return r8Flags, r8Deps
 }