Turn DesugaredLibraryMismatchDiagnostic into a warning

Some build setups cause the DesugaredLibraryMismatchDiagnostic error.
However, the builds are equivalent with classic desugar builds, which
do not have this type of test.

RELNOTES: None
PiperOrigin-RevId: 371871743

src/tools/android/java/com/google/devtools/build/android/r8/Desugar.java

@@ -25,7 +25,9 @@
 import com.android.tools.r8.D8Command;
 import com.android.tools.r8.Diagnostic;
 import com.android.tools.r8.DiagnosticsHandler;
+import com.android.tools.r8.DiagnosticsLevel;
 import com.android.tools.r8.StringResource;
+import com.android.tools.r8.errors.DesugaredLibraryMismatchDiagnostic;
 import com.android.tools.r8.errors.InterfaceDesugarMissingTypeDiagnostic;
 import com.google.common.collect.ImmutableList;
 import com.google.devtools.build.android.Converters.ExistingPathConverter;
@@ -424,6 +426,14 @@ public void warning(Diagnostic warning) {
       }
       DiagnosticsHandler.super.warning(warning);
     }
+
+    @Override
+    public DiagnosticsLevel modifyDiagnosticsLevel(DiagnosticsLevel level, Diagnostic diagnostic) {
+      if (diagnostic instanceof DesugaredLibraryMismatchDiagnostic) {
+        return DiagnosticsLevel.WARNING;
+      }
+      return level;
+    }
   }
 
   private void desugar(