[One .NET] $(AndroidEnablePreloadAssemblies)=False by default (#5790)

Context: e0da1f1

The [`$(AndroidEnablePreloadAssemblies)`][0] property controls
whether or not *all* `.dll` files contained within a `.apk` are
loaded during process startup.  *Not* doing so reduces process
startup times, which is desirable, but this also caused certain
Xamarin.Forms apps to fail to run as they previously had, as
not loading all assemblies during startup broke their
Dependency Injection infrastructure.

For .NET 6, we feel we have *some* "wiggle-room" to change default
semantics, so for .NET 6 projects set the default value of
`$(AndroidEnablePreloadAssemblies)` to False, so that assemblies are
*not* pre-loaded during process startup.

`$(AndroidEnablePreloadAssemblies)` can be set to True within the
app's `.csproj` file to return to "legacy" semantics.  This will
cause all assemblies to be loaded during startup, with a
commensurate increase in app startup overheads.

[0]: https://docs.microsoft.com/en-us/xamarin/android/deploy-test/building-apps/build-properties#androidenablepreloadassemblies

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs

@@ -3932,7 +3932,11 @@ public void PackageNamingPolicy ([Values ("LowercaseMD5", "LowercaseCrc64")] str
 				Assert.IsTrue (b.Build (proj), "build should have succeeded.");
 				var environment = b.Output.GetIntermediaryPath (Path.Combine ("__environment__.txt"));
 				FileAssert.Exists (environment);
-				Assert.AreEqual ($"__XA_PACKAGE_NAMING_POLICY__={packageNamingPolicy}", File.ReadAllText (environment).Trim ());
+				if (Builder.UseDotNet) {
+					Assert.AreEqual ($"__XA_PACKAGE_NAMING_POLICY__={packageNamingPolicy}{Environment.NewLine}mono.enable_assembly_preload=0", File.ReadAllText (environment).Trim ());
+				} else {
+					Assert.AreEqual ($"__XA_PACKAGE_NAMING_POLICY__={packageNamingPolicy}", File.ReadAllText (environment).Trim ());
+				}
 			}
 		}
 

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -290,7 +290,8 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 	<AndroidMakeBundleKeepTemporaryFiles Condition=" '$(AndroidMakeBundleKeepTemporaryFiles)' == '' ">False</AndroidMakeBundleKeepTemporaryFiles>
 
 	<!-- If true it will cause all the assemblies in the apk to be preloaded on startup time -->
-        <_AndroidEnablePreloadAssembliesDefault>True</_AndroidEnablePreloadAssembliesDefault>
+	<_AndroidEnablePreloadAssembliesDefault Condition=" '$(UsingAndroidNETSdk)' == 'true' ">False</_AndroidEnablePreloadAssembliesDefault>
+	<_AndroidEnablePreloadAssembliesDefault Condition=" '$(UsingAndroidNETSdk)' != 'true' ">True</_AndroidEnablePreloadAssembliesDefault>
 	<AndroidEnablePreloadAssemblies Condition=" '$(AndroidEnablePreloadAssemblies)' == '' ">$(_AndroidEnablePreloadAssembliesDefault)</AndroidEnablePreloadAssemblies>
 	<_NativeAssemblySourceDir>$(IntermediateOutputPath)android\</_NativeAssemblySourceDir>
 	<_AndroidUseNewTypemaps>True</_AndroidUseNewTypemaps>