gradle.properties: Enable D8 bytecode desugaring.

This is mainly to work around this issue:
https://issuetracker.google.com/u/0/issues/72872257

gradle.properties

@@ -1 +1,2 @@
 org.gradle.jvmargs=-Xmx2048M
+android.enableD8.desugaring=true

oeffi/proguard.cfg

@@ -32,6 +32,8 @@
     public static <fields>;
 }
 
+-dontwarn java.lang.invoke.**
+
 # serializable
 -keepclassmembers class * implements java.io.Serializable {
     static final long serialVersionUID;