Multiple improvements to the data hub:

* Making registration finer-grained so we only listen to the data we really need.
* Moving the system listener registration to an external class (for testability and readability)
* Adding a unit test (not yet complete)
* Adding documention and cleaning up comments

Still many things TODO:
* Allow pausing of registrations
* Add pause/resume to onPause/onResume of activities
* Move sampling logic out of the hub
* Optimize data reloading when display preferences change
* (maybe) move the data hub and related classes to the "content" package

SignalStrengthSamplerTest/AndroidManifest.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+      package="com.google.android.apps.mytracks.signalstrength.test"
+      android:versionCode="1"
+      android:versionName="1.0">
+  <application android:label="Signal Strenght Sampler Test">
+    <uses-library android:name="android.test.runner" />
+  </application>
+  <uses-sdk android:minSdkVersion="3" />
+  <instrumentation android:targetPackage="com.google.android.apps.mytracks.signalstrength"
+                   android:name="android.test.InstrumentationTestRunner" />
+</manifest>