Fix GetFreeMemory to use fooprint limit instead of max memory.

Based on definitions in:
http://developer.android.com/reference/java/lang/Runtime.html

Bug: 15507122
Change-Id: I02f34682d7ac2d379a07631b5207b6cfb224da6b
diff --git a/runtime/gc/heap.cc b/runtime/gc/heap.cc
index d7b673e..f5f7a86 100644
--- a/runtime/gc/heap.cc
+++ b/runtime/gc/heap.cc
@@ -2464,7 +2464,7 @@
 }
 
 size_t Heap::GetPercentFree() {
-  return static_cast<size_t>(100.0f * static_cast<float>(GetFreeMemory()) / GetMaxMemory());
+  return static_cast<size_t>(100.0f * static_cast<float>(GetFreeMemory()) / max_allowed_footprint_);
 }
 
 void Heap::SetIdealFootprint(size_t max_allowed_footprint) {