Update compile SDKVersion from 25 to 27

Fixes #750 

Currently, Keyman for Android targets SDK version 25 (Nougat). 

This PR targets  the latest Android API and updates all the Gradle dependencies to the latest. This will remove some lint Obsolete warnings.

Sample and Test projects needed to add 
```
implementation 'com.android.support:support-v4:27.1.1'
```
to ensure all dependencies use the same version.