Changes needed for gradle 3.0.1 compatibility

[pacamara]

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: status-im/status-legacy#3037