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

[Berstanio]

For now I have build r8/d8 somewhere on my disk following the instructions here: https://r8.googlesource.com/r8/+/d8-1.1/README.md

[Berstanio]

Now r8 gets build together with moe. r8 would need to be cloned to external/r8 by repo, I haven't tested this yet.
To get r8 build efficently and without the chromium depot_tools, a minimal patch would need to be applied to r8. This doesn't effect the resulting jar.

diff --git a/buildSrc/src/main/java/tasks/DownloadDependency.java b/buildSrc/src/main/java/tasks/DownloadDependency.java
--- a/buildSrc/src/main/java/tasks/DownloadDependency.java	(revision 82beb3099fab513a2da769a449a61f0d27e6c3ad)
+++ b/buildSrc/src/main/java/tasks/DownloadDependency.java	(date 1658436230329)
@@ -46,9 +46,9 @@
   }
 
   public void setDependency(String dependency) {
-    outputDir = new File(dependency);
-    tarGzFile = new File(dependency + ".tar.gz");
-    sha1File = new File(dependency + ".tar.gz.sha1");
+    outputDir = getProject().file(dependency);
+    tarGzFile = getProject().file(dependency + ".tar.gz");
+    sha1File = getProject().file(dependency + ".tar.gz.sha1");
   }
 
   @InputFiles
@@ -108,7 +108,7 @@
     public void run() {
       try {
         if (type == Type.GOOGLE_STORAGE) {
-          downloadFromGoogleStorage();
+          //downloadFromGoogleStorage();
         } else if (type == Type.X20) {
           downloadFromX20();
         } else {