ART: Add GetClassSignature

Add support to retrieve the signature of a JNI class.

Add a test.

Bug: 31684578
Test: m test-art-host-run-test-912-classes
Change-Id: I1cf946996f9089cb5d41e93b4f1064193e9d6350
diff --git a/test/Android.bp b/test/Android.bp
index 37e1fc0..af70486 100644
--- a/test/Android.bp
+++ b/test/Android.bp
@@ -255,6 +255,7 @@
         "909-attach-agent/attach.cc",
         "910-methods/methods.cc",
         "911-get-stack-trace/stack_trace.cc",
+        "912-classes/classes.cc",
     ],
     shared_libs: [
         "libbase",