Update Android build dependencies

[friederbluemle]

This updates build tools to 25.0.2, Gradle wrapper to 3.4.1, and the plugin to 2.3.0 (latest versions as of Mar 23, 2017)

It also replaces android-apt with the native Gradle directive for annotation processing (this is now possible with a more recent version of Gradle. No need for a custom plugin anymore).

Note that it also re-adds the default gradle.properties file. This file should be under version control to set project-wide settings (e.g. java heap space). Without increasing the max heap space, dex would run out-of-process, slowing down the build (and generating a warning).

It also removes the hard-coded Java 7 statements from EventBusTest. This is not necessary and even results in an error with the latest build tools (Java 8 is required).

_{^{Please NO GitHub rebase/squash merges. If you'd like me to combine some or all of the commits, please let me know, and I will update my branch. Thank you.}}

[greenrobot-team]

Thanks for your efforts, but we generally update dependencies ourselves when necessary. -ut

[friederbluemle]

@greenrobot-team No worries.. Anything problematic or any concerns you see with this PR? I could split it up into more than one PR if that helps.

[friederbluemle]

Commits amended and PR updated with latest versions Gradle wrapper 3.4.1 and Android Gradle plugin 2.3.0.