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

build.gradle

@@ -8,9 +8,10 @@ buildscript {
             url 'https://maven.google.com/'
             name 'Google'
         }
+        google()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.0.0'
+        classpath 'com.android.tools.build:gradle:3.1.0'
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files