Call .rs.dtor() when tearing down Scripts.

BUG=5186750

This allows us to properly reference count any globals (static or extern) that
need to potentially be cleaned up.

Change-Id: I03d2c38c1e7a4ca96c40003d2eeecb6f395d5835
diff --git a/rsScript.cpp b/rsScript.cpp
index f62c72e..93513fe 100644
--- a/rsScript.cpp
+++ b/rsScript.cpp
@@ -72,6 +72,12 @@
     mRSC->mHal.funcs.script.setGlobalObj(mRSC, this, slot, val);
 }
 
+bool Script::freeChildren() {
+    incSysRef();
+    mRSC->mHal.funcs.script.invokeFreeChildren(mRSC, this);
+    return decSysRef();
+}
+
 namespace android {
 namespace renderscript {