Mark libexpat as double_loadable

libexpat is a VNDK lib and also used by LLNDK(libmediandk) which means
this lib can be double-loaded.
(deps: libmediandk -> libmedia -> libexpat)

Bug: 121280180
Test: m -j
Change-Id: I71c882646482f9c17abf02ca278d95e7a1242612
diff --git a/Android.bp b/Android.bp
index b3bef06..1a7842d 100644
--- a/Android.bp
+++ b/Android.bp
@@ -7,6 +7,7 @@
     vndk: {
         enabled: true,
     },
+    double_loadable: true,
     host_supported: true,
     unique_host_soname: true,
     sdk_version: "9",