Refactor Logger and add Settings-UI

- Refactored logger infrastructure, so that sensors have to provide
proper timestamps for the logged data, instead of the Logger generating
a timestamp when it is logged to the file.
- Added a settings UI where active sensors and activities can be
configured

app/src/main/res/values/dimens.xml

@@ -2,4 +2,5 @@
     <!-- Default screen margins, per the Android Design guidelines. -->
     <dimen name="activity_horizontal_margin">16dp</dimen>
     <dimen name="activity_vertical_margin">16dp</dimen>
+    <dimen name="fab_margin">16dp</dimen>
 </resources>