ART: Speed up thread stack tracing for ANRs

Share a BacktraceMap for all threads being dumped, which speeds up
ANR dumping. Results from Nexus 9 (average of five):

Before: 0.587s
After:  0.206s

Change-Id: Ia70e0dbd39049318c02de561e7b95258d4849467
diff --git a/runtime/runtime_linux.cc b/runtime/runtime_linux.cc
index f0b3c4e..122dcb1 100644
--- a/runtime/runtime_linux.cc
+++ b/runtime/runtime_linux.cc
@@ -41,7 +41,7 @@
  public:
   explicit Backtrace(void* raw_context) : raw_context_(raw_context) {}
   void Dump(std::ostream& os) const {
-    DumpNativeStack(os, GetTid(), "\t", nullptr, raw_context_);
+    DumpNativeStack(os, GetTid(), nullptr, "\t", nullptr, raw_context_);
   }
  private:
   // Stores the context of the signal that was unexpected and will terminate the runtime. The