Add `$ANDROID_HOST_OUT/apex` to the file set removed by target `installclean`.

Before this change, host APEX (`.zipapex`) packages would still be
there after `m installclean`.

Test: Run `m installclean` and check that host APEX packages are removed.
Change-Id: I446286ce700f12666be5aa90fbe3bbc648abd280
diff --git a/ui/build/cleanbuild.go b/ui/build/cleanbuild.go
index bfe2c36..0a2b510 100644
--- a/ui/build/cleanbuild.go
+++ b/ui/build/cleanbuild.go
@@ -87,6 +87,7 @@
 	// otherwise we'd have to rebuild any generated files created with
 	// those tools.
 	removeGlobs(ctx,
+		hostOut("apex"),
 		hostOut("obj/NOTICE_FILES"),
 		hostOut("obj/PACKAGING"),
 		hostOut("coverage"),