Refactor memory measurement

Move out all the memory measurement to a separate class that can cache
all of its data through orientation changes.

Tweak the display to make it draw large squares for the legend instead
of a 1x1 square. Add padding to the percentage bar chart.

Change-Id: I4cd5bc4ba31a422a55740e8f58e5e571cf9866a5

res/values/attrs.xml

@@ -42,4 +42,9 @@
         <!-- Radius of the shadow. -->
         <attr name="android:shadowRadius" />
     </declare-styleable>
+
+    <declare-styleable name="PercentageBarChart">
+        <!-- Background color -->
+        <attr name="emptyColor" format="color" />
+    </declare-styleable>
 </resources>