Updating to API level 15.

Main justification: the SDK for level 15 comes with the source to the core libraries, making debugging much easier.

MyTracks/libs/README.txt

@@ -0,0 +1,22 @@
+The libraries contained here in binary format are licensed according to their
+own licenses, which allow their usage for the My Tracks application. Any other
+applications that wish to use these files should consult the appropriate
+licenses or the JAR's authors.
+
+google-api-client/
+- see http://code.google.com/p/google-api-java-client/.
+- contains Google API client library, google-api-client-1.5.0-beta.jar, and its
+  dependencies.
+- google-api-client-1.5.0-beta.jar depends on protobuf-java-2.2.0.jar, however,
+  MyTracksLib already includes protobuf-java.2.3.0-lite.jar. Thus using the
+  newer version, protobuf-java.2.3.0-lite.jar.
+- google-api-client-1.5.0-beta.jar also depends on xpp3-1.1.4c.jar, however,
+  some of its classes are already included in the android framework, thus not
+  including xpp3-1.1.4c.jar.
+
+google-common.jar
+
+googleloginclient-helper.jar
+
+libGoogleAnalytics.jar
+- see http://code.google.com/mobile/analytics/docs/.