Rewrite backtrace-helper to use Unwinder directly.

Test: ./art/test.py -b -r --host --gcstress
Change-Id: Ifdb45c9508cf359386d3de451c6c286d0f40d8f3
diff --git a/runtime/Android.bp b/runtime/Android.bp
index 2d15e4a..d73d93f 100644
--- a/runtime/Android.bp
+++ b/runtime/Android.bp
@@ -400,6 +400,7 @@
         "liblog",
         // For common macros.
         "libbase",
+        "libunwindstack",
     ],
     static: {
         static_libs: ["libsigchain_dummy"],