Fix enable_desugar logic reversal in java_library_impl

Make it so targets that set enable_desugar=false end up with desugar
disabled, instead of still enabled. This affects at least the following
targets that tried to disable desugar but it didn't work until now:
//third_party/byte_buddy
//third_party/accessibility_test_framework
//third_party/google-truth

Change-Id: I074c26e988311cc6d04dc0173da676f33b225283
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437376
Commit-Queue: Tomasz Śniatowski <tsniatowski@vewd.com>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811726}

build/config/android/internal_rules.gni

@@ -3555,7 +3555,7 @@ if (enable_java_templates) {
             "$target_out_dir/$_output_name.processed.jar"
         _dex_path = "$target_out_dir/$_main_target_name.dex.jar"
         _enable_desugar =
-            !defined(invoker.enable_desugar) || !invoker.enable_desugar
+            !defined(invoker.enable_desugar) || invoker.enable_desugar
       }
 
       # For static libraries, the javac jar output is created at the intermediate