Fix building app under Android Studio 3

This PR allows to build TagTime under the latest stable version of Android Studio (3.1.2 as of now).

Here is what had to be done:

Update gradle wrapper to 4.5
Update gradle plugin to 3.1.2
Disable aapt2 to fix a build error in ActionBarSherlock
Upgrade compileSdkVersion to 18 to make it consistent with the version of the support library that was already in use
Remove explicit buildToolsVersion. A default version is provided by recent gradle plugin versions.

NB: Running with instant run causes a build error from aapt. To fix that error, you'll have to disable instant run in Android Studio.

tagtime / TagTime