Update gradle build to use latest tools, check style and allow signing

These changes update the gradle build to:

1. Use the most recent versions of the SDK platform and build tools
2. Add a style checker to the build process
3. Allow the apk being output to be signed

My personal favourite way to build and output a signed apk, which allows the build process to be scripted without having to save your keystore or keyalias passwords in plaintext, is like this:

```
./gradlew android:assembleRelease \                                                                                                                                                                                 
    -Pandroid.injected.signing.store.file=$KEYFILE \                                                                                                                                                                
    -Pandroid.injected.signing.store.password=$KEYFILE_PASS \                                                                                                                                                           
    -Pandroid.injected.signing.key.alias=$KEYALIAS \                                                                                                                                                               
    -Pandroid.injected.signing.key.password=$KEYALIAS_PASS
```