Add low memory mode option to ART.

Useful so that we match the option I added here:
https://googleplex-android-review.googlesource.com/#/c/328940/

In ART low memory mode reduces the maximum number of histogram
buckets. We also trim no matter the utilization.

Change-Id: I655ba63312c0a6574569cdd5171ca81ea338c2aa
diff --git a/runtime/runtime.h b/runtime/runtime.h
index d480e36..47fff29 100644
--- a/runtime/runtime.h
+++ b/runtime/runtime.h
@@ -97,6 +97,7 @@
     size_t heap_max_free_;
     double heap_target_utilization_;
     size_t stack_size_;
+    bool low_memory_mode_;
     size_t lock_profiling_threshold_;
     std::string stack_trace_file_;
     bool method_trace_;