)

Fixes: dotnet#6211

Mono VM will return a valid AppDomain pointer (both in "legacy" and
NET6 cases) only if the current thread is attached to some domain.
It is possible that when managed code is called from an unattached
Java thread, `mono_domain_get()` will return `nullptr` instead of a
valid one.  If we further pass `nullptr` to other Mono functions, a
segfault may occur if Mono fails to validate the passed pointer.

Sample code which may trigger the problem:

	public override void OnCreate()
	{
	   AppDomain.CurrentDomain.UnhandledException += (sender, e) => {
	       Console.WriteLine("!!! UnhandledException: " + e.ExceptionObject.GetType().FullName);
	   };
	   base.OnCreate();
	}

	protected override void OnStart()
	{
	    base.OnStart();
	    Java.Util.Concurrent.Executors.NewSingleThreadExecutor()
	        .Execute(new Runnable(() => {
	            throw new Exception("Exception from Java Executor!");
	        }));
	}

Possible crash caused by the above code:

	F libc    : Fatal signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0xb8 in tid 19241 (pool-1-thread-1), pid 19214 (ndroidcrashtest)
	F DEBUG   : backtrace:
	I hwservicemanager: getTransport: Cannot find entry android.hardware.graphics.mapper@3.0::IMapper/default in either framework or device manifest.
	F DEBUG   :       #00 pc 0011b72f  /apex/com.android.runtime/lib/bionic/libc.so (pthread_mutex_lock+31) (BuildId: 471745f0fbbcedb3db1553d5bd6fcd8b)
	F DEBUG   :       #1 pc 0015240d  /data/app/com.companyname.androidcrashtest-sqgk-Mnu8GZM5hKPu1yWEQ==/lib/x86/libmonosgen-2.0.so (mono_domain_assembly_search+61)

	F libc    : Fatal signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0xb8 in tid 19763 (pool-1-thread-1), pid 19737 (ndroidcrashtest)
	F DEBUG   : backtrace:
	F DEBUG   :       #00 pc 0011b72f  /apex/com.android.runtime/lib/bionic/libc.so (pthread_mutex_lock+31) (BuildId: 471745f0fbbcedb3db1553d5bd6fcd8b)
	F DEBUG   :       #1 pc 0026b91a  /data/app/com.companyname.androidcrashtest-JQoYc3YFwZtEoSJlTqX_BA==/lib/x86/libmonosgen-2.0.so (mono_os_mutex_lock+42)

To avoid the above situation, wrap the `mono_domain_get()` call into
`utils.get_current_domain()` which checks the return value of
`mono_domain_get()` and, if it's `nullptr`, calls
`mono_get_root_domain()` and attaches the current thread to that
root domain.