Use r8 for dexing and backporting (MOE 1.x)

[Berstanio]

I reordered r8 task and validator task so that the validator runs first.
For the moment I didn't changes variable/task names, because it was just a POC. I will do this later.
This currently uses d8, to try to use r8 just replace here and in the sdk publisher all occurence of "d8.jar" with "r8.jar" and adjust line 266 in Proguard.java to use the proguard config. In the moe-core the proguard configs needs to be adjusted that 1. "-ignorewarn" isn't used and 2. "-keepcompanionclasses" needs to be removed.
But r8 will currently fail with a NPE. It seems to be because of the processing of the moe-core.jar, but I don't know why nor have I found some way yet to print debug infos.

Related pr's are: multi-os-engine/moe-sdk-publisher#4, multi-os-engine/moe-tools-classvalidator#2, multi-os-engine/moe-core#4, multi-os-engine/moe-tools_master#3 and multi-os-engine/manifest#2
This fixes multi-os-engine/multi-os-engine#171

[Berstanio]

r8 fails when the resolve of the super class of "java.lang.Object" returns null. Adding three simple null checks to r8 solves the issue.
https://github.com/Berstanio/r8/commits/main
Currently r8 strips to much, but this should be fixable.
However when I disable shrinking, the App just doesn't start correctly.
//EDIT: Okay, for some reason "-dontoptimize" and "-dontshrink" still strips attributes. Now that I have fixed it, a test app runs fine!