Mark libarect and libmath as vendor_available

They are used by libui which is VNDK.

Bug: 37731063
Test: sailfish builds and boots
Test: BOARD_VNDK_VERSION=current m libui is successful
Change-Id: I7ecd9b482f8055aaa159e539d630d0034f4b7074
diff --git a/libs/arect/Android.bp b/libs/arect/Android.bp
index 0d25176..a20154f 100644
--- a/libs/arect/Android.bp
+++ b/libs/arect/Android.bp
@@ -23,5 +23,6 @@
 cc_library_static {
     name: "libarect",
     host_supported: true,
+    vendor_available: true,
     export_include_dirs: ["include"],
 }