[ci] Don't convert Javadoc to Xmldoc for PR builds

Context: #5253 (comment)

When Javadoc-to-Xmldoc conversion was enabled, `make jenkins` took
1h 19min 47sec, up from 34min 45sec (ouch!).

Disable Javadoc-to-Xmldoc on PR builds.  Hopefully `make jenkins`
times will return to normal.

Author: jonpryor

build-tools/automation/azure-pipelines.yaml

@@ -67,6 +67,7 @@ variables:
   DotNetNUnitCategories: '& TestCategory != DotNetIgnore & TestCategory != AOT & TestCategory != MkBundle & TestCategory != MonoSymbolicate & TestCategory != PackagesConfig & TestCategory != StaticProject & TestCategory != Debugger'
   NUnit.NumberOfTestWorkers: 4
   GitHub.Token: $(github--pat--vs-mobiletools-engineering-service2)
+  CONVERT_JAVADOC_TO_XMLDOC: $[ne(variables['Build.DefinitionName'], 'Xamarin.Android-PR')]
 
 # Stage and Job "display names" are shortened because they are combined to form the name of the corresponding GitHub check.
 stages:

src/Mono.Android/Mono.Android.csproj

@@ -25,8 +25,7 @@
   </PropertyGroup>
 
   <PropertyGroup>
-    <!-- TODO: Default to False; ~doubles the `make jenkins` build time -->
-    <IncludeAndroidJavadoc Condition=" '$(IncludeAndroidJavadoc)' == '' And '$(AndroidFrameworkVersion)' == '$(AndroidLatestStableFrameworkVersion)'">True</IncludeAndroidJavadoc>
+    <IncludeAndroidJavadoc Condition=" '$(IncludeAndroidJavadoc)' == '' And '$(CONVERT_JAVADOC_TO_XMLDOC)' == 'true' And '$(AndroidFrameworkVersion)' == '$(AndroidLatestStableFrameworkVersion)'">True</IncludeAndroidJavadoc>
   </PropertyGroup>
 
   <PropertyGroup Condition=" '$(IncludeAndroidJavadoc)' == 'True' ">