Update D8 check_file with check_exe

Adds a dependency on JDK 16.0.1 so that the version of D8 can be checked
with `java -cp <jar> com.android.tools.r8.D8 --version` instead of just
checking that the jar exists.

This change also simplifies the targets section.

bin/yaml/android-java.yaml

@@ -2,8 +2,10 @@ compilers:
   android-d8:
     type: singleFile
     dir: r8-{name}
+    depends:
+      - compilers/java 16.0.1
+    check_exe: "%DEP0%/bin/java -cp {dir}/{filename} com.android.tools.r8.D8 --version"
+    filename: r8-{name}.jar
+    url: https://dl.google.com/android/maven2/com/android/tools/r8/{name}/r8-{name}.jar
     targets:
-      - name: 8.1.56
-        filename: r8-8.1.56.jar
-        check_file: r8-8.1.56.jar
-        url: https://dl.google.com/android/maven2/com/android/tools/r8/8.1.56/r8-8.1.56.jar
+      - 8.1.56