[Xamarin.Android.Build.Tasks] <GenerateJavaStubs/> should open readonly in some cases

[jonathanpeppers]

I was investigating a failure building Android projects in Release mode inside VS Mac:

Task "GenerateJavaStubs"
    Failed to read '/usr/local/share/dotnet/packs/Microsoft.NETCore.App.Runtime.Mono.android-arm/8.0.0-preview.5.23280.8/runtimes/android-arm/lib/net8.0/Microsoft.CSharp.dll' with debugging symbols. Retrying to load it without it. Error details are logged below.
    System.UnauthorizedAccessException: Access to the path '/usr/local/share/dotnet/packs/Microsoft.NETCore.App.Runtime.Mono.android-arm/8.0.0-preview.5.23280.8/runtimes/android-arm/lib/net8.0/Microsoft.CSharp.dll' is denied.
    ---> System.IO.IOException: Permission denied
    --- End of inner exception stack trace ---
    at Interop.ThrowExceptionForIoErrno(ErrorInfo errorInfo, String path, Boolean isDirError)
    at Interop.CheckIo(Error error, String path, Boolean isDirError)
    at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, UnixFileMode openPermissions, Int64& fileLength, UnixFileMode& filePermissions, Func`4 createOpenException)
    at System.IO.Strategies.OSFileStreamStrategy..ctor(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
    at Mono.Cecil.ModuleDefinition.ReadModule(String fileName, ReaderParameters parameters) in /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/cecil/Mono.Cecil/ModuleDefinition.cs:line 1103
    at Java.Interop.Tools.Cecil.DirectoryAssemblyResolver.LoadFromMemoryMappedFile(String file, ReaderParameters options) in /Users/runner/work/1/s/xamarin-android/external/Java.Interop/src/Java.Interop.Tools.Cecil/Java.Interop.Tools.Cecil/DirectoryAssemblyResolver.cs:line 184
    at Java.Interop.Tools.Cecil.DirectoryAssemblyResolver.ReadAssembly(String file) in /Users/runner/work/1/s/xamarin-android/external/Java.Interop/src/Java.Interop.Tools.Cecil/Java.Interop.Tools.Cecil/DirectoryAssemblyResolver.cs:line 169
    /usr/local/share/dotnet/packs/Microsoft.Android.Sdk.Darwin/34.0.0-preview.5.312/tools/Xamarin.Android.Common.targets(1513,3): error XA0009: Error while loading assembly: '/usr/local/share/dotnet/packs/Microsoft.NETCore.App.Runtime.Mono.android-arm/8.0.0-preview.5.23280.8/runtimes/android-arm/lib/net8.0/Microsoft.CSharp.dll'.
Done executing task "GenerateJavaStubs" -- FAILED.

I noticed DirectoryAssemblyResolver.LoadFromMemoryMappedFile() was not opening the file in read-only mode for Debug builds.

In 8bc7a3e, we added "marshal methods" support that unconditionally enables ReaderParameters.ReadWrite.

We should be able to toggle these settings only when "marshal methods" is enabled:

var readerParams = new ReaderParameters();
if (useMarshalMethods) {
    readerParams.ReadWrite = true;
    readerParams.InMemory = true;
}

This should improve performance for Debug builds, and avoid opening files in read/write mode.