Enable arm64-v8 in app.gradle

[darind]

Starting from August 1, 2019, all new apps and app updates that include native code are required to provide 64-bit versions in addition to 32-bit versions when publishing to Google Play:

https://android-developers.googleblog.com/2019/01/get-your-apps-ready-for-64-bit.html

We should therefore include the arm64-v8 architecture in the application gradle so that newly created applications get the 64 bit runtime by default.

[vhristov5555]

Arm32 is faster than Arm64 on Arm64 devices. The results are recorded on Pixel 2 device.