Update Gradle wrapper to 4.6

[friederbluemle]

This PR also includes the build script updates that matches the new Gradle version (previous Gradle updates only update the text file gradle/wrapper/gradle-wrapper.properties without doing a full update using the gradle wrapper command). Included is also the (now standard) increase in heap space that allows dex to run in process, fixing the following warning:

Running dex as a separate process.

To run dex in process, the Gradle daemon needs a larger heap.
It currently has 1024 MB.
For faster builds, increase the maximum heap size for the Gradle daemon to at least 1536 MB.
To do this set org.gradle.jvmargs=-Xmx1536M in the project gradle.properties.
For more information see https://docs.gradle.org/current/userguide/build_environment.html

_{^{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.}}

[smalichenko]

@friederbluemle Two questions:

1. Any reasons to update gradle wrapper to 3.5? Currently android projects use 3.3 wrapper version https://developer.android.com/studio/releases/gradle-plugin.html
2. Also, i don't think that increasing heap size inside the plugin is a good idea. Plugin, itself don't need a lot of memory. And, it's just a plugin, it's not a standalone android project. If you need to increase heap size for your android project, you can do it directly for the project, but not for plugin.

[friederbluemle]

Gradle 3.5 brings performance improvements. And as explained above, without the increase in heap space, dex will run out of process, slowing down the build, and generating the warning. It's also the default now.

[friederbluemle]

Rebased onto latest master branch.

[friederbluemle]

Commit amended and updated to the latest stable version 4.0.
@smalichenko Do you have any other questions or concerns about this?

[friederbluemle]

Updated again to latest stable version 4.0.1

[friederbluemle]

Updated again to latest stable version 4.1

[friederbluemle]

Updated again to latest stable version 4.2.1 and rebased onto latest master branch.

[friederbluemle]

Updated again to latest stable version 4.3.1

[friederbluemle]

Updated again to latest stable version 4.6