Changes needed for gradle 3.0.1 compatibility

Some minor changes are needed for gradle 3.0.1 compatibility: in android/build.gradle:
* Remove the `buildToolsVersion`, gradle now specifies a default and the current value conflicts with that.
* Declare dependencies using `api` not `compile` per https://developer.android.com/studio/build/gradle-plugin-3-0-0-migration.html#new_configurations

We are currently hacking these changes with a script in status.im, and would rather it is fixed upstream. Thanks!

Parent issue: https://github.com/status-im/status-react/issues/3037

Start in android/build.gradle and compare its buildToolsVersion and dependency configurations with the Gradle 3.0.1 migration guidance linked in the issue. Done means removing the conflicting buildToolsVersion and using api instead of compile so the upstream project no longer needs the compatibility script.

Vom Indexierungsmodell aus dem Issue-Text verfasst.